Cross-Sections of a 12-mm. Dolphin Embryo
By
HIROSHI HOSOKA W A1 )A 12-mm. embryo of a dolphin (Prodelphinus caeruleo-albus, Meyen)shown in Fig. 1 was embedded in celloidin, sectioned transversely andserially, 15 µ in thickness. Every other section of the series was
stained with hematoxylin and eosin; and studied microscopically.
In the following, some of the important levels as indicated by
guide lines on Fig. 2 are illustrated or photographed, together withbrief descriptions. All sections are seen from the cephalic side. Hencethe right side of the embryo is on the left side of figures.
There have been published only a few literatures upon the fetal
development of whales or cetacea, among which Guldberg and Nansen'sbook (1894) is probably the most important and extensive one. They
dealt mainly with the external features of dolphin fetuses (Lagenorhynchus acutus, Gray; Lagenorhynchus albirostris, Gray; Phocaena
communis, Lesson; Orea gladiator, La Cepede; etc.), accompanied bysome microscopic observation of partial sections.
In the present study, serial sections of a human embryo of some
10-mm. in length as well as of pig embryos of 11 and 13-mm. wereused for comparison. Several books of embryology helped the observations, too. Among the referred are such books as written by Arey
(1946), Baxter (1953), Boyden (1947), Grosser & Politzer (1953), Huettner (1952), Patten (1927, 48, 53), etc.
Fig. 3. Section through the Isthmus Rhombencephali and Pontine
Flexure (Serial No. 57).
Because of the pontine flexure, the head is cut twice at this level.
The section passes through the mesencephalon, isthmus rhombencephali
and metencephalon on the anterior half, and the myelencephalon is cuton the posterior half. The cavity within the mesencephalon, theforerunner of the aquaeductus mesencephali, is still large, retaining thefeature of the mid-brain vesicle. The ventriculus quartus is cut apartinto two portions, each of which being covered by a very thin ependymal
roof on one side.
1) from the Department of Anatomy (Director: Prof. Dr. T. Ogawa), University ofTokyo Faculty of Medicine, Hongo, Tokyo.
Scient. Repts. Whales Res. Inst. 10 (1955).

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H. HOSOKAWA

The wall of the neural tube has differentiated into three successive
layers; (1) an inner, ependymal layer, (2) a middle, mantle layer, and(3) an outer, marginal layer (Plate I, 1). The ependymal layer, thecontributor of neuroblasts and neuroglia to the mantle layer, is provided
with multitudes of cells which are arranged radially. In the innermost zone of cthis layer, abutting on the internal limiting membrane,
many mitotic cells are distinguished (Plate I, 2, 3). The mantle layer,
which is to make up the gray substance in future, contains many cellbodies of the primitive neurons and neuroglial cells. The marginallayer, the progenitor of the white substance, is noncellular, consistingof processes of the mantle-layer cells or of the peripheral sensory
cells.
The outer surface of the neural tube is surrounded by a thin,mesenchymal layer. This is the primitive pia mater or leptomeninx
and its rich blood vessels in the area of the ependymal roof will
participate in forming the choroid plexus.
In the mesenchyme between the neural tube and the epidermis
are distributed many blood vessels, some of which are branches of theinternal carotid or of the basilar artery, while others represent tributaries of the anterior cardinal vein (anterior and medial dural plexus).
The tip of the endolymphatic duct is cut along both sides of theventral end of myelencephalon. The tiny section of the trochlear nerve
is also noticed on each side of the rhombic isthmus.
Fig. 4. Section through the Fourth Ventricle (Ser. No. 69).
The fourth ventricle is sectioned nearly horizontally above its
middle height. Due to the pontine flexure and probably also due tothe shrinkage by the fixation, two fragments of its roof are cut.
Among new structures shown at this level are the rootlets of thenervus oculomotorius and of the n. accessorius. The former fibers are
just leaving the wall of the mesencephalon.
The pricipal three-layered structure of the neural tube is a little
rearranged in the anterior portion and the mantle layer is transformed
to make up the primitive nucleus n. oculomotorii. Posterior to this isa part of the trochlear nucleus, too. A few of the mitotic cells arescattered in these primitive nuclei, although the mantle layer cells
usually do not show cell-division. The primitive nuclei may be better
regarded as outwards migrating cells of the ependymal layer.
The root fibers of the accessory nerve, emerging from the wall of
the myelencephalon as well as of the spinal portion, form a somewhat
thick trunk and run anterolaterad to join the vagus nerve.
Fig. 5.

Section through the Apex of an Otocyst (Ser. No. 95).

Cross-sections of a 12-mm. Dolphin Embryo

3

On the right half of the section the apex of the left otocyst is cut
laterally to the endolymphatic duct, while only a shaving of the leftotocyst is in view on the opposite side.
The wide cavity occupying almost the middle one third of thesection represents the metencephalic portion of the fourth ventricle, of
which the inner surfaces of both sides bear rhombic grooves (III-VI) as
well as the intervenient prominences, rhombomeres or neuromeres (IIIVI).
Because of the cervical flexure, the anterior greater part of the
myelencephalon is cut through the floor plate, although its posterior end
is seen well as it approaches the spinal cord.
Alongside the myelencephalon are rootlets of the nervus hypoglossus,
anterior to which the n. vagus leaves the brain wall. The accessory
nerve is now adjacent the vagus trunk.
In the interval between the mesencephalon and metencephalon are
the cut oculomotor nerves, surrounded by the anterio1· dural plexus.
The left trochlear nerve is not grown down to this level, while the tipof the right one is still see!)..
Fig. 6.

Section through the Floor of Metencephalon (Ser. No. 109).
The section passes through the floor of the metencephalic portionof the fourth ventricle, leaving a narrow space on the median line.
The fundamental three-layered structure of the neural tube is distorted
not a little around here. For the nerve fibers begin to extend in all
directions in the mantle layer, dividing the latter into more or lessisolated cellular masses, which will become the so-called nuclei. Also
the mixture of the gray and white substances, the formatio reticularis,
will be formed.
About midway along the metencephalon is then. statoacusticus, justissuing from the brain wall and directed to the otocyst. This nerve iscomposed of two nerves, the nervi cochleae and vestibuli.
The endolymphatic duct is now annexed to the otocyst. Posterior to
the latter are the nn. glossopharyngicus and vagus, together with theganglia intracraniale and jugulare respectively. The cells in the jugular
ganglion are said to be sensory in function and to send fibers to theauricular branch of the vagus nerve. No trace of this tiny branch is,
however, noticed in this dolphin embryo.
The spinal cord is cut obliquely at the level of the first cervical
ganglion, although the corresponding ventral root does not occur yet.
Along the inner margin of the spinal ganglion is the tiny cut of the
spinal accessory nerve, which can be traced a little more distance
below the present level.

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H. l-IOSOKAWA

Considerable number of mitotic cells are identified in the spinal
ganglia (Plate I, 4). The same holds good for the semilunar ganglion
as well as for the nodosoal, jugular, extra- and intracranial, sympathetic
ganglia.
·Fig. 7. Section through the Infundibulum, Ganglia Semilunare andGeniculi (Ser. No. 135).
The metencephalon has disappeared and the wide interval between
the two cuts of the neural tube, the diencephalon and spinal cord, is
occupied by the mesenchymal area.
On the midplane of this area are the notochord and arteria basialis,both of which cut lengthwise. The unpaired basilary artery is formedby the fusion of the paired vertebral arteries. On each side of thebasilary artery is the n. abducens, which has emerged from the floorof metencephalon and is directed anterolatero-caudad to the primitiveeye muscles.
Anterior to the otocyst in the middle of the section is the ganglion
associated with the statoacoustic nerve. The n. facialis, situatedlateral to the ganglion just mentioned and cut longitudinally, bears
the gangUon geniculi on its anterior end.
The ganglion semiluna1·e stands out as a large, oval cell masscircumscribed sharply. The portio minor of the trigeminal nerve isattached to the inner side of this ganglion.
The vascular space between the semilunar and geniculate ganglion
represents the vena capitis prima that is the main tributary of theanterior cardinal vein and receives the anterior as well as the medial
dural plexus.
The anterior cut of the neural tube, the diencephalon, has a
midventrally directed pouch, the 1:nfundibulum, which will furnish theposterior lobe of the hypophysis.
Between the facial nerve and spinal cord, named in order, are then. glossopha1·yngicus, anterior cardinal vein (vena jugularis interna),
nn. vagus and accessorius, n. hypoglossus, the first cervical nerve andthe vertebral artery. The lst cervical nerve is seen as it divides into
the vental and dorsal rami, surrounded by the thick, mesenchymal cellmass destined to form the cervical muscles.
· Fig. 8. Section through the Fornix Pharyngis, Ganglia Extracraniale and Nodosum (Ser. No. 145).
New structures occurring on the present section are the fornix
pharyngis, the ganglion extracraniale of the glossopharyngeal nerve,
the cranial end of the ganglion nodosum of the vagus nerve, the 1st

Cross-sections of a 12-mm. Dolphin Embryo

5branchiat groove, the 1st pharyngeal pouch, the nervus pet'rosus superjicialis ma.for, the primitive eye muscles, a bit of the Rathke's pouch,etc.
The n. petrosus superjicialis major, whose existence in the adultwhales or dolphins is not yet ascertained, is represented distinctly inthe present fetus by a tiny nerve bundle that has arisen from the
geniculate ganglion of the facial nerve and runs anteromedio-caudad.
The 1st branchial groove, the future external acoustic meatus, iscut midway along each side of the embryo. Althogh it seems to becontinuous with the lateral end of the 1st pharyngeal pouch lined withthick epithelium, this communication must be due to the artificial
rupture of the closing membrane.
The Rathke's pouch which has originally budded from the stomodeal
epithelium and is destined to furnish the glandular hypophysis showson this section as two cuts that flank the infundibulum on both sides.
The semilunar ganglion is bordered mediad by the cavernous sinus.
Anteromedial to these structures is the accumulated mesenchymal massrepresenting the primordial eye muscles, nn. abducens and oculomotorius
being located within it.
Fig. 9. Section through the Optic Cup (Ser. No. 153).
Inside the pharyngeal cavity is a shaving of mesenchymal tissue
lined with epithelium, which represents the root of tongue.
Anterior and posterior to the pharynx are two cuts of thenotochord, the anterior one of which is close to its cranial end.
The facial nerve located lateral to the pharynx has just given off
a small branch, the chorda tympani, which is, like the n. petrosus
superficialis major, not yet ascertained in the adult whales or dolphins.
The 1st pharyngeal pouch and the inte1·nal carotid artery are cut
lengthwise, the latter being crossed externally by the n. petrosus
superjicialis maJ·or.
The semilunar ganglion has disappeared at this level and two
branches of the trigeminal nerve, nn. maxillaris and mandibularis,
have occured. The mesenchymal condensation surrounding the latter
nerve represnts the primordium of the chewing muscles.
The Rathke' s pouch shows now as a vesicle, of which the anterior
and posterior wall will furnish respectively the anterior lobe and thepars intermedia of hypophysis (Plate II, 5).
Anterolateral to the primordial eye muscles is the cut optic cupthat is a double-walled vesicle now. The inner thick layer of thisvesicle is the forerunner of the retina and the outer thin coat, theprogenitor of the pigment epithelium, is provided with pigment granules

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H. HOSOKAWA

already (Plate II, 6).
Among new structures appeared on the dorsal half of this sectionis the sympathetic nerve which is situated close to the vagus nerve.
Traced caudad it joins the ganglion cervicale craniale of the sympathetic
trunk.
Fig. 10. Section through the Root of Tongue and Lens Vesicle(Ser. No. 163).
This section passes approximately 150 µ below the previous level.
What marks this level is the wide pharyngeal cavity with the cut firstand second pharyngeal pouches extending respectively anteromediad andposterolaterad. Needless to say, the former pouch is to furnish the
tympanum as well as the tuba pharyngotympanica, while the latter one
foreshadows the tonsillar sinus.
The root of tongue shows as a large, separate mass m thepharyngeal cavity. Its anterior portion must be the tuberculum impar
and the middle, lateral expansions may represent the copula.
Posterior to the 2nd pharyngeal pouch are nn. glossopharyngicus,
vagus and hypoglossus. The nerve bundle cut lengthwise laterally tothe left vagus nerve is the rwrnus externus of the accessory nerve. On
the right side the whole accessory nerve is still accomanying the vagus
trunck. The cranial cervical ganglion of the sympathetic trunk has comeinto view on the dorsal side of the vagus.
In the anterior part of the section, n. maxillaris is diminishing insize considerably. The optic vesicle is cut nearly through its center andtherefore the lens vesicle is sectioned on this plane. A thin ectodermal
layer, the corneal epithelium, borders outside.
Fig. 11. Section through the Origin of Rathke's Pouch, Epiglottis
and Optic Stalk (Ser. No. 173.)
Characteristic of this section is the division of the pharyngeal
cavity into two parts. The anterior one is the oral cavity or stomodeum,of which the epithelium is ectodermal. Although the oral (or pharyngeal)
membrane marking the boundary between the ectodermal and entodermal
epithelium has disappeared at this stage, the Rathke's pouch that hasarisen just in front of this membrane is shown to continue with theanterior wall of the oral cavity on the midline.
The pharynx is now a transverse slit compressed antero-posteriorly
and the prominence boundering its anterior wall represents theepiglottis. Antero-laterad to this pharyngeal slit is the cut apex of
the 2nd pharyngeal pouch.
Flanking the vagus trunk mediad are two arteries, of which theCross-sections of a 12-mm. Dolphin Embryo

7anterior and the posterior are respectively the 3rd aortic arch and theinternal carotid artery.
The glossopharyngeal nerve has given rise to a small branch, n.
tympanicus, to the antero-medial direction. The main trunk of thisnerve is diminishing in the size.
The optic stalk, the connection between the optic cup and the
diencephalon, is a thin tube, of which the lumen continues with thethird ventricle on one side and with the space between the inner andouter layer of the optic cup on the other side.
Section through the Foramen Interventriculare, Oral
Opening, 3rd Pharyngeal Pouch and Glottis (Ser, No. 187).
Fig. 12.

The telencephalon is cut at the level of the foramen interventiculare
connecting the 3rd ventricle with the lateral ventricle. So the cerebral
hemisphaerium is cut near its caudal end.
The oral cavity is now open to the external through the slit between
the maxillary and mandibular process. The n. mandibularis in themandibular process is still within view as tiny rootlets, while the n.
maxillar·is in the maxillary process does not reach this level.
The pharynx is an inverted V-shaped slit, of whicp. the both feet
represent the 3rd pharyngeal pouches. The apex of this pouch isprovided with two wings, the ventral and the dorsal, that are to furnish
the thymus and the parathyroid gland respectively (Plate II, 7). Also
the fourth pouch participates in this formation.
The triangular space located on the midline anterior to the pharynx
represents the cranial portion of the larynx, while the linear, epithelial
connection between the pharyngeal and laryngeal lumen is the primitive
glottis. This is bordered on each side by a pair of the arytenoidswellings, which are said to be of the fourth and fifth arch origin.
The 3r·d aortic arch has shifted its position anterior to the 3rdpharyngeal pouch. Needless to say, it has crossed above the latter.
Anterior to this aortic arch is the external carotid artery, the cranial
extension of the ventral aorta. The dorsal aorta is located posterolateral
to the pharynx, being flanked posteriorly by the sympathetic trunk.
Fig. 13. Section through the 4th Aortic Arch, 4th Pharyngeal
Pouch and Thyroid Gland (Ser. No. 203).
On each side of the pharynx and larynx which are now separated
completely from each other the 4th aortic arches are cut lengthwise.
Obviously the left one is larger than the right one. The anterior andposterior end of this arch are continuous with the ventral and dorsal
aorta, although the right aortic arch meets the right ventral aorta

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H. HOSOKAWA

slightly below the present level. The ventral aorta at this level is theprogenitor of the common carotid artery.
Between the ventral aortae on each side is a mass of cell cords
that represents the primordial thyroid gland (Plate II, 8). This isoriginally an entodermal pocket invaginating from the ventral floor of
the pharynx, although the epithelial connection between this gland andthe pharynx has disppeared already. The site of origin was in the
midplane at the level of the lst pharyngeal pouches.
The pharynx is sectioned through the proximal portion of the 4thpharyngeal pouch. A portion of the primordial parathyroid derived
from the dorsal wing of this pouch is also seen. The mesenchymal cellmass surrounding the larynx foreshadows the laryngeal cratilages andmuscles.
Lateral to the 4th aortic arch are the n. vagus, n. hypoglossus andvena jugularis interna. The hypoglossal nerve has continued to shift
ventrad and is now cut lengthwise as it courses ventromedio-caudad.
The ramus communicans connecting between the spinal nerve and thesympathetic trunk is seen on the left side.
On the left side of the embryo a proximal portion of the upper
limb bud is coming into view. The mandibular nerve in the same-named
process is now beginning to disappear.
Fig. 14. Section through the Ventral Aorta and Primitive Choana(Ser. No. 215).
The section passes through the paired dorsal aortae and the single
ventral aorta. The right dorsal aorta is, together with the right fourth
aortic arch, to form the right subclavian artery, while the left one isto persist as the cranical portion of the permanent descending aorta.
The ductus arterious, the sixth aortic arch of the left side, is also cut
as it joins the left dorsal aorta. The unpaired ventral aorta is the
cranial extension of the aortic trunk. It may be called the forerunnerof the ascending aorta.
The pharynx and larynx are sectioned close to the caudal ends.
Both of them are traced caudad respectively to the oesophagus andtrachea. Ventromedial to the larynx is a portion of the primordial
thymus which belongs to the ventral wing of the fourth pharyngeal
pouch.
Nerve rootlets situated between the larynx and the dorsal aorta
represent the n. recurrens, a branch of the vagus nerve. The vagus
trunk is located laterally tog·ethet with the caurtal end of the ganglion
nodosnm.
The mandibule is now separate from the body of the embryo andCross-sections of a 12-mm. Dolphin Embryo

9retains no trace of the mandibular nerves.
The head is cut through the nasal vesicle and the primitive choana.
The latter is the site of the ruptured bucco-nasal membrane.
Fig. 15. Section through the Apex of Pericardia! Cavity (Ser. No.
223).
The section is marked by the appearance of the pericardial cavity
as well as of tip of the heart. In the left portion of the pericardia!
cavity is a shaving of the left atrium, within which are many sinusoids
filled with blood. The atrial wall and the trabeculae partitioning thesinusoids are provided with primitive muscles.
When the ventral aorta shown on the preceding section is traced
caudad, it joins the aortic trunk which at the present level makes aprominence into the pericardia! cavity.
On the midplane posterior to the pericardial cavity are, named inorder, the trachea and oesophagus lined with thick epithelium. The
mesenchymal masses surrounding these tubes foreshadow the connective
tissues, cartilages or muscles of the tracheal and oesophageal walls.
The right vagus nerve is sectioned at the level of the caudal endof the nodosal ganglion. The branching of the right recurrence nerve
and the right cardiac rami are shown, too. On the left side a portionof the ductus arteriosus or the left sixth aortic arch is still in view.
The upper limb bud is encountered on each side of the embryo : anerve bundle belonging to the brachial plexus is inside the left one.
The right and left mandibular processes are cut apart from eachother.
In the head the fila olfactoria make connection between the brain
wall and the epithelium of the nasal vesicle. The medial wall of thenasal vesicle lined with thick epithelium represents the Jacobson'svomero-nasal organ.
Fig. 16. Section through the Pulmonary Trunk (Ser. No. 235).
What characterises this section is the pulmonary trunk or thecornmon pulmonary artery cut lengthwise on the left side of the aortic
trunk. The distal or posterior end of this trunk divides into the rightand left pulmonary arteries that have originated from the ventral halvesof the 6th aortic arches. A little distance above the present level, thedorsal half of this arch or the connection between the pulmonary arteryand the dorsal aorta is preserved well on the left side as the ductus
arteriosus, while it has disappeared on the right side. On the anteriorof the pulmonary trunk is the primordial semilnnar 'valve.
Medial to the anterior ca1·dinal veins which are now :flowing into

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H. HOSOKAWA

the common cardinals are the vagus nerves. This nerve on each side
has just given rise to the tracheal and oesophageal rami caudad.
Fig. 17. Section through the Bulbus Cordis, Common Cardinal
Veins and Origin of Right Apical Bronchus (Ser. No. 249).
The section passes through the undivided bulbus cordis, in which
the lumen of aorta and common pulmonary artery are connected witheach other through a narrow slit. The parenchyme masses abutting on
this intervening slit on both sides represent the ventral and dorsal
bulba1· ridges, which have shifted position to the right and left sideres pee ti v ely.
The apex of the pleiiral cavity has come into view on each side of
the trachea. The latter is just giving off the right apical or eparterial
bronchus. Ventral to the trachea are the paired pulmonary arteries.
The pleural cavity is continuous with the pericardia! cavity throughthe pleuro-pericardial commiinication which is guarded laterally by thepulmonary ridge of Mall.
Lateral to the pleural cavity is a large space representing thecommon cardinal vein or the duct of Cuvier. This is approaching thesinus venosus now.
The upper limb bud is sectioned on each side of the embryo,provided with mesenchymal condensations foreshadowing the futurebones and muscles. Also the cranial end of brachial plexus as well as
the phrenic nerve is encountered on the section.
The head is cut through the fronto-nasal process, retaining only
tips of the nasal vesicles inside. The cerebral hemisphaeria are lost tosight.
Fig. 18. Section through the Foramen Ovale I, Bifurcatio Tracheae
and Nasal Pit (Ser. No. 261).
What marks this level is the foramen ovale I that is a perforationin the septum primum. Thus the right and left atrial chamber are
partitioned incompletely from each other at this level. The rightcommon cardinal vein has emptied into the right horn of the siniis
venosus, while the connection between the sinus and the right atrium
is to occur a little more caudad. The right vertebral vein is seen as it
flows into the right common cardinal.
The root of the bulbus cordis is cut together with the apexes ofright and left ventricles. In the center of the cut bulb is thetriangular space of the aorta, while pulmonary trunk has just joined
the right ventricle. The slit between the ventricles and artia represents
the transverse sinus.

Cross-sections of a 12-mm. Dolphin Embryo

11

Posterior to the heart, on the midline, is the bifurcatio tracheae,
and the cut apical bronchus is on each side. The bronchial buds in theprimordial lung are continuing to grow and branch. They are linedwith thick, ciliated epithelium at the present stage (Plate III, 9). The
mesenchymal mass surrounding the bronchial buds is to furnish thecartilages, muscles, vascular channels as well as the interstitial tissues
for the lung.
The head is sectioned close to its rostral end, and the nasal pits or
the primitive external nasal orifices are shown at this level.
Fig. 19. Section through the Sinus Venosus (Ser. No. 273).
The hear·t is marked by the sinus venosus (right horn), which now
drains into the right atrium through a silt-like opening. The projections
guarding this opening on both sides represent the right and left valvesof the sinus venosus. The left common cardinal is located in the pericardia! cavity. Caudad it is traced to the left horn of the sinus
venosus. The septum primum is complete at this level.
The ventricles have increased in size, the aorta being sectioned on
the midplane. In the right ventricle a portion of the atrioventricular
canal is cut obliguely. In the thich ventricular wall three layers canbe discriminated, 1) the external, pericardial layer, 2) the middle,
myocardial layer, and 3) internal, endocardial layer. The external
furrow marking off the ventricles from the atria is the coronary
sulcus.
In the primordial lung the left main bronchus is seen to give off
craniad the left apical bronchus, of which the distal cut was observed
already on the preceding section.
Among other structures which mark this level is the apex of theperitoneal cavity, which lies antero-lateral to the left dorsal aorta.
Posterior to the dorsal aortae which are still separate from each otherare the vertebral arteries and the postcar·dinal veins.
Fig. 20.
287).

Section through the Trunk of Pulmonary Vein (Ser. No.

The section passes through the trunk of the pulmonary vein, which
drains into the postero-caudal wall of the left atrium at the left of the
septum primum. Later, this venous trunk will be progressively drawn
into the left atrium, as the latter grows.
The septum primum is attached to the endocardial cushion. The
structure just mentioned is originally endocardial thickenings whichbulged from the ventral and dorsal walls of the common atrio-ventricular
canal and have fused midway, dividing the single canal into the right12

H. HOSOKAWA

and left one.
The sinus venosus (right horn) as well as some of the trabeculae
are shown in the right atrium. The sinal valves are elongated ventrocaudad. The right valve will remain even in the adult heart, being
subdivided and forming the crista terminalis, the Eustachian valve of
the inferior vena cava as well as the Thebesian valve of the coronary
sinus. The left valve is merely to fuse with the septum primum andsecundum, in order to form a portion of the permanent atrial septum.
The internal cavity of the left ventricle has come into view,connected with the left atrium through the left atrio-ventricular canal,
the primordium of the left atrio-ventricular ostium. This canal isguarded on each side by the cut endocardial cushions which foreshadow
the mitral valve. The opening of the aorta into the left ventricle isalso seen.
The thick, muscular partition between the ventricles represents
the septum interventriculare. Posteriorly it joins the endocardial
cushion.
Postero-lateral to the left atrium is attached the left horn of thesinus venosus, which has received the left common cardinal vein. This
sinal horn is destined to disappear excepting its tip that is to furnish
the stem of the Marshall's vein.
The peritoneal cavity occurs on each side, being separated fromthe pleural and pericardia! cavity by the pleuro-peritoneal membrane
and the septum transversum respectively. Both of these partitions will
participate in forming the diaphragma, in cooperation with derivaties
from the body wall and a part of the meso-oesophagus. At the leftof the left phrenic nerve the diaphragmatic muscle is foreshadowed bya mesenchymal condensation.
Fig. 21. Section through the Anastomosis between Right and Left
Dorsal Aortae (Ser. No. 293).
Most characteristic of the section is the confluence of the rightand left dorsal aortae, thus forming the unpaired descending aorta.
The peritoneal cavity has increased in size and a shaving of the leftpart of the liver occurs on the left side. In the primordial diaphragma
the left phrenic nerve is cut close to its peripheral end, while theright one is still considerably large.
The heart is sectioned through the transverse portion of the sinus
venosus, which is destined to receive the veins of the heart itself andis, roughly speaking, foreshadows the sinus coronarius.
The right hm·n of the sinus venosus is seen to receive the cephalicend of the common hepatic vein. The connection between the trans-Cross-sections of a 12-mm. Dolphin Embryo

13verse portion and the right horn of the venous sinus is, however,not in view. It occurs slightly caudal to the present level.
Fig. 22. Section through the Pleuro-peritoneal Communication(Ser. No. 303).
The section is marked by the distinct appearance of the liver oneach side as well as of the comrrwn hepatic vein on the miplane. The
heart is cut through its basis and shows, excepting the caudal extensionof the transverse portion of the sinus venosus, only the dorso-caudal
walls of the ventricles.
The partition between the pleural and peritoneal cavity by thepleura-peritoneal membrane is now incomplete on each side. For thesetwo cavities are connected through the intervening foramen, thepleura-peritoneal communication. The latter increases in size as thesections are traced caudad.
The vagus nm·ves adjacent to the oesophagus have shifted position
to some extent, the left one moving to the ventral surface of theoesophagus and the right one dorsad. It means that the alimentary
canal itself has rotated clockwise around its longitudinal axis.
Fig. 23. Section through the Ductus Venosus (Ser. No. 315).
Ventral half of the section is occupied by the liver, although aslice of the heart is still in view, restricted to the ventral border ofthe embryo. The liver nearly fills the peritoneal cavity and its
parenchyme is permeated with network of cell cords and the intervening
sinusoids (Plate III, 10). The former is a spongework of the primordial
hepatic cells with oval nuclei and is stained lighter than the sinusoids,
while the latter, filled with proliferative blood cells such as megaloblasts,
normoblasts, myeloblasts, etc., is stained darker. Nucleated erythrocytes
are found also in the circulating blood (Plate III, 11).
Even though traced the whole length of the liver, it does notshow any sign of lobulation, while it is plainly lobulated in the humanand pig embryos of the similar size.
The large vescular space in the center of the liver represents theductus venosus, which receives several efferent veins from inside theliver and continues craniad with the common hepatic vein.
The right vagus nerve dorsal to the oesophagus bears peculiarly
several ganglion cells at the present level. The lung buds approach
their caudal ends and the mesonephric ridges have occured posterior
to the former. Furthermore, the cranial tip of the left mesonephrosor the Wolffian body is already in view.
Fig. 24.

Section through the Stomach (SeL No. 341).

14

H. HOSOKAWA

The section is marked by the appearance of the stomach, bursa
omentalis, vena cava inferior, genital ridges, etc., while the lung andheart are now lost to sight.
The stomach is a dilated portion of the fore-gut, which is connectedwith the liver and the dorsal body wall by the onwntum minus(gastrohepatic ligament) and the dorsal mesogastrium (omentum majus)
respectively. The internal surface of the stomach is lined with entodermal epithelium, although the foveolae and the gastric glands are
not yet indicated.
The vena cava inferior is cut slightly below its confluence with theductus venosus. Traced caudad .it takes its course dorsad through thecaval mensentery. The narrow space flanked by the caval mesentery,liver, omentum minus and the stomach represents the rostral extensionof the bursa omentalis or the lesser peritoneal sac.
On each side of the descending aorta is the distinctive mesonephros.
Together with secretory and collecting tubules, a glomerulus is also
seen. The aorta supplies it with the mesonephric artery. Ventromedial to the mesonephric ridges occur the genital ridges.
The ventral attachment of the liver to the body wall is themesohepaticum ventrale, on both sides of which the mesothelial liningof the liver reflects to the internal surface of the body wall.
Fig. 25. Section through the Adrenal Cortex and Vestibule ofOmental Bursa (Ser. No. 363).

Because of the rotation of the stomach about its long axis, theoriginal ventral wall (lesser curvature) is on the right side and theprimitive dorsal wall (greater curvature) is on the left. The cut
omental bursa is necessarily enlarged at this level, bordered behind by
the elongated dorsal mesogastrium. The right portion of the bursa
reprents the vestibule and it is bounded on the right side by the caval
mesentery, within which the inferior vena cava passes.
On each side between the aorta and the mesonephros is the adrenal
cortex. It is round in shape and consists of remarkable condensationof mesenchymal cells (Plate III, 12). This is not the primordium of
the permanent cortex but represents the provisional or fetal cortex
that is later to be replaced by cells of another type to furnish thepermanent cortex. Both of these two kinds of cells are, however,thought to be traced origin to the same proliferative focus. The
chromaffine cells to form the adrenal medulla are not yet identified.
Very soon they will differentiate from the sympathetic ganglia andinvade the medial side of the cortical mass.
Inside the liver the umbilical vein and the portal vein are cut asCross-sections of a 12-mm. Dolphin Embryo

15

they flow into the ducts venosus.
Fig. 26. Section through the Foramen Epiploicum and Splenic
Primordium (Ser. No. 371).
The Section passes through the foramen epiploicum which connects
the omental bursa (vestibule) with the peritoneal cavity. Because ofthis communication the inferior· vena cava is separate from the liver,
although it still runs in the caval mesentery. Right subcardinal vein
located just medial to the right mesonephros is seen to join the inferior
vena cava.
The latero-dorsal surface of the dorsal mesogastrium shows a slightswelling which represents the primordial spleen. This swelling is dueto the accumulated mesenchymal cells just beneath the lining, peritoneal
epithelium (Plate IV, 13). Later on the splenic bulge increases in size
and the mesenchymal mass will differentiate into red and white pulp,
vascular channels, capsule, trabeculae, etc.
The mesonephros increases in size, too, and the mesonephric duct or
Wolffian duct is cut inside the ventro-lateral margin of the mesonephricridge. The Miillerian duct is, however, not identified.
Fig. 27. Section through the Root of Coeliac Artery (Ser. No. 379).
The aorta is cut as it gives off the unpaired coeliac artery andthe paired intersegmentary arteries ventrad and dorsad respectively.
These are originally ventral and dorsal intersegmentary branches of
the aorta. The lateral branches are represented by the mesonephricwrteries.
The portal vein is now shifting into the omentum minus and thehepatic duct is also seen anterior to the just mentioned vein.
The adrenal cortex and the primordial spleen are in almost thesame condition as in the previous section. The vascular supply of thelatter is through the splenic artery and vein, of which the former is abranch of the coeliac artery and the latter is a tributary of the portal
vein.
The gonad in the genital ridge becomes larger as it is traced
caudad. The gonad at this stage is indifferent or sxless. It's minutestructure consists of internal cell mass lined with thick, germinal
epithelium (Plate IV, 14, 15).
Fig. 28. Section through the Primordial Pancreas (Ser. No. 401).
The liver is cut near its caudal end, the right and left part being
separate from each other. In the intervening space between these
two is the umbilical vein, which is fastened to the ventral body wall

16

H. HOSOKAWA

by the jalciform ligament.
The most conspicuous feature appearing on this section is, however,the primordium of the pancreas in the mesoduodenum (Plate IV, 16).
It consists of two portions, the ventral and the dorsal pancreas. Bothare originally outpouchings from the entodermal epithelium of theduodenum, and they are contorted as they grow. The duct of theventral pancreatic primordium is shown at its union· with· the hepatic
duct (antrum), thus froming the common duct for the bile and thepancreatic juice.
The gall bladder is not recognisible all through the series
concerned, while it is well developed in the human and pig embryosin the similar stage of the f eial development. Therefore the lack ofgall baldder must be characteristic of the whale or dolphin embryos aswell as of the adult whales and dolphins.
The portal vein is now in the mesoduodenum, situated between
the ventral and the dorsal pancreas. The inferior vena cava has, as
already mentioned, fused with the right subcardinal vein which islocated just medial to the right mesonephros. The left subcardinal isin view, too. The narrow spaces along the dorsal border of themesonephroi represent the postcardinal veins.
Fig. 29. Section through the Exit to Superior Mesenteric Arteryand Umbilical Cord (Ser. No. 419).
What marks the present level is the fusion of the umbilical cordwith the fetus. The tip of the recurved tail of the embryo is also cut
near the anterior border of the figure.
Concerning the body of the fetus itself, tha mesenteric connection
between the alimentary canal and the liver has disappeared, thus
leaving only the dorsal mesentery or mesoduodenum. In this area theduodenum is cut just before its union with the common bile duct.
The splenic vein has jointed the portal vein already. So thecaudal continuation of the portal vein at the present level represents
the vitelline vein. The confluence of the superior mesenteric vein into
the vitelline occurs still a little distance below the present section.
The umbilical vein is located in the ventral body wall of the fetus.
It is large, being sectioned at the union of the right and left umbilicalvein.
The cut umbilical cord shows such structures as the paired umbiUcal
arteries, unpaired urackus, and the extra-fetal coelomic extension. Inside
the last mentioned cavity are the shaving of the tip of the loop of
herniated small intestine, the superior mesenteric vein as well as the
rudimentary vitelline vein.

Cross-sections of a 12-mm. Dolphin Embryo

17

Fig. 30. Section through the Subcardinal Anastomosis (Ser. No.
443).
The section is characterised by the subcai·dinal anastomosis or thewide communication between the right and the left subcardinal vein.
The postcardinals on both sides are also continuous with the anastomosis.
The vitelline vein cut somewhat lengthwise on the left of the small
intestine is prominent into the peritoneal cavity. Traced caudad it
becomes separate from the mesentery and runs independently in the
coelom, although it receives the superior mesenteric vein while it isstill in the mesentery. The distal portion of the vitelline is seen in
the coelomic extension in the umbilical cord.
Inside the coelomic extension tl:\e small intestine is cut twice, theright section and the left one representing the cranial and caudal
limb of the intestinal loop respectively.
Fig. 31. Section through the Coelomic Extension into Umbilical
Cord (Ser. No. 469).
The section passes through the extra-fetal extension of the peritoneal cavity (coelom) into the umbilical cord. Within this cavity is
the ·instestinal loop in the condition of the physiological hernia. The
cranical and caudal limb of the loop are on the right and the left
respectively. The former portion is the small intestine, while thelatter may represent the colon.
The protrusion of the intestinal canal into the umbilical cord isthought to be caused by so rapid elongation of the intestine that theperitoneal cavity can no longer contain it. In case of the humanfetus, the withdrawal of the hernia occurs at about ten weeks of the
fetal life.
Along the intestinal canal are the superior mesenteric artery andvein. The latter is a tributary of the vitelline vein, which is nolonger seen on the section.
In the cut tail of the fetus such features as spinal cord, notochord,somite and caudal artery are identified.
Fig. 32. Section through the Caudal Limb of Intestinal Loop (1)(Ser. No. 485).
In the center of the wide coelom protruding into the umbilicalcord is the caudal limb of the intestinal loop (colon) and is connected tothe dorsal body wall by the elongated mesentery (mesocolon). The
right umbilical vein is peculiarly cut as it extends into the umbilical cord.
The mesonephros as well as the gonad is shown at about the largest
section. The mesonephros or the Wolffian body consists of glomeruli,

18

H. HOSOKAWA

mesonephric tubules, mesonephric duct (Wolffian duct), vascular channels
and intervening mesenchymal tissue (Plate V, 17). Needless to say,the mesonephric duct is the retained pronephric duct. It is lined withdark-staining, cuboidal epithelium.
In the perplexingly convoluted mesonephric tubules two different
portions or segments can be discriminated. One is the thicker, lightstaining secretary segment and the other is the thinner, dark-staing
collecting segment. The former is proximal to the glomerulus and isnothing but the direct, tubular extension of the Bowman's capsule. The
Bowman's capsule itself is, however, lined with low epithelial cells.
The collecting segment is the distal continuation of the secretory
segment and drains into the mesonephric duct.
Fig. 33. Section through the Caudal Limb of Intestinal Loop (2)(Ser. No. 501).
All the structures and features are almost the same as in theprevious figure, excepting that the mesentery is somewhat shortened
and the superior mesenteric blood vessels have disappeared.
The urachus or the allantoic stalk is an irregular, slit-like spacewith epithelial lining of cuboidal cells. This epithelial lining is entodermal. For the allantois is originally an outpouching from the ventral
floor of the primitive gut, although it had furthermore an externallayer of splanchnic mesoderm.
Figs. 34 & 35. Sections through the Root of Umbilical Cord (1, 2)(Ser. No. 525 & 541).
These two figures show sections through the root of the umbilicalcord at different levels. So far as the body of the embryo is concerned,
there are no remarkable differences from a couple of preceding sections.
The only thing to be mentioned is that the gonads now dwindle awayand disappear.
In the umbilical cord, which is decreasing in size, are umbilical
blood vessels, urachus, etc. The coelomic extension is shown as one or
two separate cavities.
The paired umbilical arteries and unpaird urachus are cut twice or
thrice, depending on their courses and the height of section. The
proximal portion of these arteries as well as of the urachus are now
along the internal surface of the ventral body wall of the fetus.
The right umbilical vein is just rudimentary, while the left one isvery large. The latter is the main venous channel carrying back theblood from the placenta.

Cross-sections of a 12-mm. Dolphin Embryo

19

Section through the Metanephroi and Inferior Mesenteric
Artery (Ser. No. 555).
The section is marked by the metanepkroi or the primordium of
the permanent kidneys. They arise from the caudalmost portion of thenepkrogenic cord, from which also the pro- and mesonephros originate.
On the section they are on each side of the aorta, just behind andsomewhat mediad to the mesonepkros.
The vetral margin of the mesonephric ridge has become prominentand the mesonepkric duct is within it, turning its course ventrocaudad.
The aorta is cut at the level of exit of the inferior mesentericartery. Like the coeliac and superior mesenteric artery this is one ofthe ventral branches of the aorta which were originally very imperfectly
segmentary. While the former two supply the stomach-pancreas region
and the small-intestine region respectively, the last one carries blood
mainly to the large-intestine region.
Fig. 36.

Fig. 37.

Section through the Metanephroi (2) (Ser. No. 565).
This section is slightly below the previous one, cutting themetanephros at about its middle height. The mesonepkros has been
lost to sight on the left, while the caudal tip of the right one is
scarcely in view.
The metanepkros is composed of two elements, 1) the tubularepithelial portion and 2) condensed mesenchymal tissue (Plate V, 18).
The former is the distal end of an evagina tion from the caudal
portion of the mesonepkric duct and represents the primitive renal
pelvis and calyces. It will further differentiate and furnish the collecting
segments of the uriniferous tubules.
The second component of the metanephros or the condensed
mesnchymal mass surrounding the epithelial tubules is the metanepkrogenic tissue in narrow sense. For it is destined to make the secretary
segments of the uriniferous tubules. The interstitial connective tissue
as well as the renal capsule will differentiate from the external layerof the same mesenchymal mass.
Fig. 38. Section through the Ureter, Genital Tubercle and Hind
Limb Bud (Ser. No. 577).
The section passes through the caudalmost portions of the metanepkroi, and the ureters descending from the former are cut ventral
in position. The epithelial lining of the ureter, which is continuouswith the epithelial tubules in the metanephros, is formed by low
cuboidal cells. A thin layer of accumulated mesenchymal mass envelopes

20

H. HOSOKAWA

it externally.
The ventral end of the section shows a peculiar prominence which
represents the genital tubercle. Of course, it is sexless at this stage
and two or three months will elaspe before the distinctive penis or
clitoris is formed.
Another new feature occuring on the present section is the budof lower extremitiyCI). As shown in several succeeding figures, too, it
is internally just a somewhat condensed mesenchymal mass. So it isuncertain, whether or not there may occur some primordial cartilages
or muscles belonging to the lower extremity before it dwindles awayin accordance with the further growth of the f etus.
Fig. 39. Section through the Primitive Bladder (Sel. No. 583).
Because of the dorsal shifting of the umbilical arteries, the coelom
is cut three times, being separated into the unpaired median and paired
lateral parts.
The postcardinal veins have also changed position
gradually to the dorsal and are now alongside the aorta.
The urachus which has been kept in the midplane of the ventral
body wall is enlarged and triangular in shape, lined with thickepithelium. It means that the cavity mentioned represents the cranial
portion of the urogenital sinus or of the cloaca. This is the primitive
bladder.

Fig. 40. Section through the Urogential Sinus and Membrane (Ser.
No. 589.)
The urogenital sinus or cloaca is cut at the level of its fusion withthe mesonephric duct. The communication between the urogenital
sinus and the ureter is shown at the same time. In other words,
the mesonephric duct and ureter make very short common stem justbefore they join the urogenital sinus. The sinus itself is connectedwith the outer surface of the genital tubercle by an epithelial cell. cordthat represents the urogenital or cloacal membrane.
Strictly speaking, the bladder and the urogenital sinus are to bediscriminated from each other, although the former is merely a cranial
continuation of the latter. The joints of the mesonephl'ic ducts andureters to the urogenital sinus mark the caudal limit of the bladder,the remaining portion representing the urogenital sinus in narrow sense.
(1) It is well known that the whale- and dolphin ietuses have rudimentary lower
extremities in their very early fetal stage. C.f. Guldberg (1894, 99), Guldberg & Nansen(1894), Kiikenthal (1895), Hosokawa (1952) and Ogawa (1953).

Cross-sections of a 12-mm. Dolphin Embryo

21

Fig. 41. Section through the Bifurcation of Aorta into Umbilical
Arteries (Ser. No. 593).
In the middle of the section is the aorta which now separates a way
into the right and left umbilical arteries. On each side of the aorta
is the postcardinal vein.
The intestinal canal (rectum) approaches its caudal end, surroundedby loop-like slit of the coelom. The ureters are lateral in position andshow nearly tangential sections due to their concurve course.
The mesenchymal portion behind the urogenital sinus is the cloacal
or uro-rectal septum, which fills the interval between the sinus and thehind-gut in the wedge-like fashion.
The section of the recurved tail of the f et us has increased insize. A couple of pairs of somites are seen alongside the cut spinal
cord and notochord.
Figs. 42 & 43. Sections through the Caudal Artery and Tail Gut(Ser. No. 601 & 603).
These two figures show two successive sections around the caudal
end of the alimentary canal. The features and structures in both arealmost the same, and the only noteworthy difference is that theformer shows the connection between the rectum and urogenital sinus(cloaca), while the latter is provided with a section of the tail gut or
the postanal gut. This is the caudalmost extention of the entodermal
canal a little beyond the cloacal membrane and is destined to dwindle
and disappear very soon.
The presence of the caudal artery in the center of these figures
may be worthy of notice. It is the caudal continuation of the aorta.
Fig. 44. Section through the Lumbal Curvature (1) (Ser. No. 655).
The section passes through the curvature of the lumbal region of
the fetus, the spinal cord being cut twice at the anterior and posterior
ends of the section. The tangential shaving of the notochord is in thecenter.
On each side of the section is a series of somites, which comprises
myotomes, sclerotomes and dermatomes. The myotomes, which aremost conspicuous in the section, are oval condensations of myoblasts ofthe mesenchymal origin. Needless to say, it is the forerunner of theskeletal muscles in regions of the neck, trunk and possibly of thelimbs.
The der·matomes, which from a layer of aggregated mesenchymal
cells under the surface epithElium, have arisen from the external
portion of each somite and will furnish connective tissue for the skin.

22

H. I-IosOKAWA

The sclerotomes have come from the ventro-medial walls of somites,
migrated, and have surrounded the notochord. Each of them, lying inpaired segmental masses, has furthermore differentiated into two parts,
the cranical looser and the caudal denser half. When they proceedfurther in development, these two parts of each sclerotome will separate
and the caudal denser half will unite with the cranial half of the next
sclerotome, thus forming the primordium of the vertebra. It is wellknown that, because of these procedures, the myotome and the vertebra
become to lie in alternation, and that this alternation makes it possible
for the myotomic muscles to move the spine.
Intersegmental arteries are cut in between the successive somites,
while the spinal, segmental nerves are within each somite.
Figs. 45 & 46. Sections through the Lumbal Curvature (2, 3) (Ser.
No. 669 & 673).
These figures show two sections of very close levels. The maindifference consists in the fact that the spinal cord is cut twice in thefirst figure, while it is cut longitudinally extending almost the whole
length of the second section.
In both of them spinal ganglia arranged serially are shown,
together with some of the ventral roots.
Other features and the arrangement of somites resemble that of theprevious figure very much, excepting that the sclerotomes have lateralprojections which lie intervening the myotomes. These projections are
possibly to be regarded as the primordia of the costal processes.

ResumeFrom the foregoing observation of serial sections, it may beconcluded that the internal structures and features of a 12-mm.
embryo of the Prodelphinus caeruleo-albus, Meyen, resemble very muchthat of human embryos of some 10-12 mm. in length (5-6 weeks infetal age) as well as that of pig embryos of the similar size.
There are, however, several remarkable differences or interesting
things worthwhile to be mentioned from the comparative anatomical
view-point. Some of them will be summarized in the following.
1) All of the primordia of cranial nerves are present already, andeven such nerves as n. petrosus superficialis major and chorda tympani,of which the existance in the adult whales is not yet ascertained,
occur distinctly just like in the human and pig embryos.
2) The gall bladder, in which the adult whales lack, is notforeshadowed at all in the present fetus.

Cross-sections of a 12-mm. Dolophin Embryo

233) As in the adult whales, the fetus does not show any indicationof hepatic lobulation, while the human and pig embroys of the similar
size have distinctly lobulated liver.
4) The hind limb buds appearing only in the early developmental
stage of whale's fetuses are represented by prominences permeatedwith accumulated mesenchymal mass inside, showing no indication of
the primitive muscles or cartilages at the present stage.
References
Arey, L. B.: Developmental Anatomy. Fifth Ed., Philadelphia & London, 1946.
Baxter, J. S.: Frazer's Manuel of Embryology. Third Ed., London, 1953.
Boyden, E. A.: A Laboratory Atlas of the Pig Embryo. Third Ed., Philadelphia, 1947.
Grosser, 0. and Politzer, G.: Grundriss der Entwicklungsgeschichte des Menschen. Vierte
Aufi., Berlin-Gottingen-Heidelberg, 1953.
Guldberg, G.: Ueber tempori:ire i:iussere Hinterfiossen bei Delphinembryonen. Verh. anat.
Gesellsch., 8. 92-95. 1894.
- - - : Neue Untersuchugen iiber die Rudimente von Hinterfiossen und die Milchdriisen
bei jungen Delphinembryonen. Internat. Monatsschr. f. Anat. Physiol., 16. 301321. 1899.
- - - and Nansen, F.: On the Development and Structure of the Whale. Part I; On
the Development of the Dolphin. Bergen, 1894.
Hosokawa, H.: On the Pelvic Cartilages of the Balaenoptera-Foetuses, with Remarks on
the Specifical and Sexual Difference. Scient. Repts. Whales Res. Inst., No. 5. 515. 1951.
Buettner, A. F.: Fundamentals of Comparative Embryology of the Vertebrates. Rev. Ed.,
New York, 1952.
Kukenthal, W.: Ueber Rudimente von Hinterfiossen bei Embryonen von Walen. Anat.
Anz., 10. 534-537. 1895.
Ogawa, T.: On the Presence and Disappearance of the Hind Limb in the Catecean Embryos.
Scient. Repts. Whales Res. Inst., No. 8. 127-132. 1953.
Patten, B. M.: The Embryology of the Pig. lst Ed., Philadelphia, 1927.
The Early Embryology of the Chick. Third Ed., Philadelphia & Toronto, 1948.
Human Embryology. 2nd Ed., New York & Toronto, 1953.

24

H. HOSOKAWA

Pontine flexure

Groove between
max. and mand. processes

Branchial groove I.
(Hyoma~dibular groove)
CervicalflexureBranch.
groove II.

Cephalic ....________
flexure~Branch.
arch II.
IHyoidal arch)
Eye

Mandibular
process

LacrimalgrooveFronto-nasal
processTail

Upperlimb bud

Umbilical
cord

Cardiohepatic
prominence
Genital tubercle
Hind limb bud

Fig. I.

I~I

12 mm. embryo of Prodelphinus caeruleo-albus.

25

Cross-sections of a 12-mm. Dolphin Embryo-------------- Fig. 3- - - - - - - - Fig. 5-------------~-------

', '\

\
\

--------\---..." '\\

- - - - - - Fig. 8\------;;;---\-- -------Fig. 10'\--------,---\-I- - - - Fig. 15

I

\

\

\

\

\

\

------\-t
\
I

I
\

I

I

I

I

-----~t----,--Fig. 20-----F~.~-----.+-i----Fig. 30
I
I

I

I

-----,--;- - - - - - Fig. 35_____ _}_--'-- ------ Fig, 40~-=-~=·==--=--=--=-;-1~/=-= -=-=-----=--=Figs; 41, 42
0-,_/ /':-...
/ I- - - - - - - ':>.>.,,----------/"-/- ---------Fig 44'"""".......

,....

...... ~:::--------:............... ///.

::::_-:=::..=:::::=..=-.:-=- --===-=--=--=--=---=--=--=-===Figs. 45, .46
Fig. 2.

Guide lines to indicate the levels of sections shown as Fig. 3-46.

26

H. HOSOKAWA

Epidermis ................

...... ......

.........

Tr. spinalis
n .. trigemini

Ependymalof fourth
ventricle

Telachorioidea
Metencephalon ----

Isthmusrhombencephali --

I

----

N. trochlearis

Mesencephalon -------

Fig. 3- Section through the isthmus rhombencephali and pontine flexure.

27

Cross-sections of a 12-mm. Dolphin Embryo_,. .... Tr. spin: n. trig.
Ependymal layer -._--.... ....

.... .-/

Mantle layer - _______- -abulated epidermis

Marginal layer - ---------

Tr. spin. n. trig.

---- -:.._-

Epidermis""'-;;;,:::-- - .......

Ependymalroof offourth ventricle
Lateral recessof fourth
ventricle

trochlearisn. trochlearisn. oculomotorii

Fig. 4.

Section through the fourth ventricle.

H. HOSOKAWA

28

N. £ccessorius(spinal root)r - - - - - - - - accessory ganglion
N. hYPoglossus

N. accessorius
N. vagus andjugular ganglion

Tip ofN. glossopharyngicus

otocyst:------ Otocyst

Tr. spin. n. trig. - - - -Endolymphatfo
ductRhombic groovtll!
and rhombomeres

Medialdural plexus- - - - P i a materN. trochlearis - - - - N. oeulomotorius --~==-

Anterior
dural plexus

Mesencephalon - - - -Fig. 5. Section through the apex of an otocyst.

Cross-sections of a 12-mm. Dolphin EmbryoMedialdural plexus

Fig. 6. Section through the floor of metencephalon.

29

30

H. HOSOKAWA

Spinal c o r d - - - - - - - -A. vertebralis

Spinal ganglion
Primitive
cervical - - - - muscles

Dorsal ram us. }Ventral ramus
of spinal nerve (C,)

Notochord----Anteriorcardinal vein - - - (Vo jUgo into)
OtocystGanglia of n.

Nn. vagus& accessorius
N. glcssopharyngicus' - - - N . facialis

statoacusticus~====

Vena capitis _ _ __prima____ Geniculate
ganglion

Ao basialis

Ant. dural _ _ __plexus----.....

Semilunar
ganglion
Portio minor.
of n. trig.

Infundibulum----Diencephalon - - - - -Fig. 7.

Branch ofa. carotis
int.

Section through the infundibulum, ganglia semilunare and geniculi.

Cross-sections of a 12-mm. Dolphin Embryo

31

N. petrosus
superf. major
Portio minorof n. trig.
A. carotis---- Semilunar ggl.

Rathke' pouch------ :<·abducens

Primordial
eye muscles

Ant. dural plexus

Fig. 8.

Section through the fornix pharyngis, ganglia extracraniale and nodosum.

32

H. HOSOKAWA

3pinal cord-----------------

Rathke'Diencephalon--------- -Fig. 9.

Section through the optic cup.

33

Cross-sections of a 12-mm. Dolphin Embryo

Spinal ggl. - - - - - -Ggl. cervicale
cranialePrimitive
dorsal mm.

N. hypoglossus

Notochord -----•..;.;

N. vagus &nodosal ggl.

V. jug. int.
A. carotis int.

Root of tongue - - Pharyngeal
pouch II - - - Pharynx----Pharyngeal
pouch I---Primitive
chewing mm.

Ramus externusof accessory nerve
Primitive sternocleidomastoideus- - - - N . glossopharyng.
r - - - - N. facialis

Primit. facial mm.
Chorda tympani- - - - N. mandibularis
N. petrosus
superf. maj.

Primitive
eye mm. - - -N. maxillaris
Lateral sinus - - - - ,Lens vesicle

Rathke' pouch - - - -Fig. 10.

Section through the root of tongue and lens vesicle.

34

H. HOSOKAWA,/,,------ A. vertebralis",//

Spinal nerves/~----

Truncussympathi<!us,/,_,,----- N. XII//•~---- N. IX in branch.

A. carotis int. --- ----3rd aortic arch---------//,,

arch III

Phary. pouch II--------------- N. VII

Branch. groove I--·------------- Chordatympani

Chewing muscles----~, ·----Tongue
Oral cavity----------- N. V,

Rathke' pouch---------- N. V,

Extrinsic
eye muscles------- - Lens vesicle',,~

Lateral sinus - - - - I

,.

.\
1

Corneal------ epithelium~--------Optic

cup

I
I

I

'--- - --- -- Optic stalkFig. 11.

Section through the origin of Rathke's pouch, epiglottis and optic stalk.

35

Cross-sections of a 12-mm. Dolphin Embryo

Spinal cord --------

Spinal ggl.

Notochord ~-­Pharynx ~1,gl
Arytenoids~

Glottis'',------ Sympath. trunk~---

Dorsal aorta
V. jug. int.
N. X &
nodosal ggl.

Larynx- - - - - - Apex ofpharyngeal
pouch III
Branch. groove I

3rd aortic arch

Mandibular-----:
process

A. carotis ext.

' - - - - - - N. V,
Maxillary _ __processr - - - - - - Oral opening

Lat. sinus - - - - - 'For amen- - - - - - - - interventriculare! ¥ - - - - - - - - Hemisphaerium

Superior
sagittal - - - - - - - '.sinus·Fig. 12.

cerebri

Section through the foramen interventriculare, oral opening, third
pharyngeal pouch and glottis.

36

H. HOSOKAWA

Branchial
groove II--- ---

B. groove I-------------Right ventral
aor::ndibular
process.;(~--~~~~~~if~/J:f.

Left ventraJ aorta
Thyroid gland

·'''

Oral

Telencephalon ------

Fig. 13.

Section through the fourth aortic arch, fourth pharyngeal pouchand thyroid gland.

37

Cross-sections of a 12-mm. Dolphin Embryo

Spinal ganglion ,Notochord ,,',,'',,,.~i/···u. '~:\·.>:~~------- .,.-__ Spinal cord'/.... '...... ....

:.,:·.;:.'.·.::::::-..-::,-.· ....

/~;u;~!r~~~ ', '1-., '-.,,Jt'·< -~:.»',,
N. recurrens ......
..............

't'',...........

IJ ..........

......

o,.,.,.
~,...

//___ Sympath.}tic trunk.,...,.,,.,.,, .... _ \----,...,.,..,

~~
~

0

.,.---c::iJ ',,---- L. dorsal aorta..-! .....

~~·--il-------~'~••-)3 ___ og J----~1¥»\ ~''1! ,/Ll --;~~-;;---------~-- _t_,YJ ~ ~::r--------~dct~:a'J~gf
)__
~ /
-----

R. dorsal aorta ----...........

',-------

V. jugularis int. -----N. hypoglossus

Spinal nerves_,/'/,.- ,.:\,.,./

• •
''Cl

a

',""'- f\,,,.,,,,.;/

Ductus arteriosus. ·,,,.. ....

Thymus----------(4th pouch)Caudal end--------of larynx
Mandibular
process

/

.....

~
- N. hypoglossus~------___./' -----~~~-------Ventral aorta//1--........_---------Primitive choana------- Nasal vesicle

Telencephalon ------Lamina terminalis

Fig. 14.

Section through the ventral aorta and primitive choana.

II

38

H. HOSOKAWA

Spinal cord

Notochord

Spinal ganglion
Spinal nerves

Oesophagus
Sympathetic trunk
R. dors. aorta

Trachea.
Brachial plexus
V. jug. int.

N. recurrens

Cardiac ramiof n. vagus
Thymus - - - - Pericardial - - - - cavity
Aortic trunk

Site of ruptured
bucco-nasal
membrane

Ductus arteriosus
Left atrium

Mandibular process

Vomero-nasal
organ

Laminaterminalis...__ _ _ _ Hemisphaerium
cerebri
Fig. 15.

Section through the apex of pericardia! cavity.

Cross-sections of a 12-mm. Dolphin Embryo----- Nasal vesicle

Hemisphaerium
cerebri

Fig. 16.

Section through the pulmonary trunk.

39

40

H. HOSOKAWA

Notochord

Spinal cordI

V. vertebralis

I
I

Oesophagus

I

I

I

I

I

R. dors.

L. dors. aorta/// N. vagus///

V. subclavia

Brachia! plexus

N. phrenicus

Common,card.

Pleural cavity
Upperlimb budR.
Pulmonary ridge
Aortic trunk

Pulmonary artery'Left atrium

Pericardia] ea vity
Nasal vesicle------· Fronto-nasal process

Fig. 17.

Section through the bulbus cordis, common cardinal veins andorigin of right apical bronchus.

41

Cross-sections of a 12-mm. Dolphin Embryo

Oesophagug
R. dors. aorta
R. vertebral vein

Notochord'\Sympath. trunk/

L. dors. aorta
N. vagus

Sinus
venosusL. atrium
Septumprim um
Pericardia!
cavityR. ventricle

ventricle

Sulcus interventriculare

Fronto-nasal process

Fig. 18.

Section through the foramen ovate I, bifurcatio tracheae and nasal pit.

42

H. HOSOKAWA

Spinal cord
Spinal nerve

Spinal ganglion

I

I
I

Syrnpath. trunk
L. dors. aorta///

Pleural cavity/

R. dors. aorta

/

/

/

/

Brach. plexus

Oesophagus---

R. mainapicalbronchiL. bronchus
giving off I.

L. commoncard. vein
L. atrium
Septum I

R. atrioventricular
canal
R. ventricle

trunk

Sulcus interventriculare

Fig. 19.

Section through the sinus venosus.

43

Cross-sections of a 12-mm. Dolphin Embryo

Spinal ganglion

Spinal cord

NotochordSympath. trunk
Post. card. vein
L .. dors. aorta
R. dors. aorta
L. lung bud
Oesophagus
Pleuroperitoneal
membrane

Peritoneal cavity

A. pulmonaris

N. vagus

N. phrenicus

V. pulmonaris

Muscle ofdiaphragm

Sinus venosus(right horn)

Peritoneal
cavityTrunk ofpulmonary vein

Valve ofsin. ven.

Left horn ofsinus venosus

Septum I
Septumtransversum

R. atrioventr. canal

L. atrium

Floor ofr. ventricle

L. atrioventr.
valve
Endocardial
cushionSeptumFig. 20.

Aorta

L. atrio-ventr. canal

Section through the trunk of pulmonary vein.

44

H. HOSOKAWA

Anastomosfo between
r. & I. dors. aortae

Notochord
Post. card.

\\

I

'

I

Liver

Right hornof sinus
venosusvcnosu~Sept. transv.

R. auricle

Pericardia! cavity

R. atrium
L. atrio-ventr. canal

R. atrioventr.
canal

Sept. interventr.
Sulc us interventr.

Transverse portion ofsinus venosus (sinus coronarius)

Fig. 21. Section through the anastomosis between right and left dorsal aortae.

Cross-sections of a 12-mm. Dolphin Embryo

Spinal cord\''

Pleura:J cavity

Pli.mrtJ
peritoneal
memb;;i;ine '-,-..,Pleuroperit.

--~oromunication '----\

Spinal ganglion\\

Post. card. vein

'

\

''

''

'

''

45

\

\

''

,,,

-lung bud

Peritoneal __ _cavityvagus

Coromonhepatic
vein

Sept. transv.

Dorsal interventr. vein
Transverse portionof sinus venosus
Fig. 22.

Section through the pleura-peritoneal communication.

46

H. HOSOKAWA

Spinal ·cord

Spinal ggl.

"

Post. card. vein

Notochord
Aorta

Sympath. trunk

Mesonephros

Mesonephric ridge

Peritoneal cavity

R. lung bud

R. vagus

L. vagus

Oesophagus

Sept. transv.
Liver
R. efferent
hepatic vein
L. efferent
hepatic vein

Sept. transv.

Fig. 23.

Pericardia! cavity

Heart(I. ventricle)

Section through the ductus venosus.

Ductus. venosus

Cross-sections of a 12-mm. Dolphin Embryo

Sympath. trunk
I

I

Notochord

\

I

I

I

I
I

Ductus venosus
LiverFig. 24.

Ramus communicims

47

H. HOSOKAWA

48

Spinal oord
Aorta
Sympath. trunk

Notochord ------·---_,/ Mesonephros

Secretory
segment-------- _ Post. card. vein

---

R.

portal vem

Vena pfortae 'going into
ductus venosus

I,/'

,,,,

j/

R. afferent
hepatic vein

L. afferent
hepatic vein

Mesohepaticum
ventraleFig. 25.

Vena umbilicalis going
into ductus venosus

Section through the adrenal cortex and vestibule of omental bursa.

.
of a 12-mm. Dolphin Embryo
Cross-sect10ns

Dorsal root
Spinal cord

49

50

H. HOSOKAWA

2mm. Dolphin Embryo
fa 1
so
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Crosss

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. Notochord
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exus
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52

H. HOSOKAWA·--- ---~}--------- Umbilical cord
Vitelline vein------------~@.....·:1-'f~ ~ 1 i--~----- C~inmio/'" a:~~~J-0 1--- ----s:.::~~~:,, ' "><"°ri~ ------~--\_,,~ 1l1""'" of lntoctlMI - - - - - - - - - \'""UrnblfolUe~bIB', ,. .,. .,. . . . . . . . --\.

"""'"''"\-D..1~--------- R. umb. vein~r---------1

umb. vein

Tail--------0·(;o._iFig. 29.

Section through the exit to superior mesenteric artery.

53

Cross-sections of a 12-mm. Dolphin Embryo

Spinal cord

N'otochord

Spinal ggJ,

Aorta

Subcardi 11al
anastomosis
Mesentery
Mesonephros
Mesonephric
duct
Small
intestine

Vitelline vein

R. umb. vein
L. umb. vein

Coelomic_....:_-- extension~
in umb. cord..·

Vitelline vein - - - - - - - ;Small intestine(cran. limb)
Sup. mesent.
vein
R. umb. arteryL, umb. vein

Urachus

Tail
Fig. 30.

R. umbilical vein

Section through the subcardinal anastomosis.

H. HOSOKAWA

54
Spinal
Ramus

trunk

Dorsal ramus---

MesoncphricarteryCaudal artery - - - - - Somite ---Fig. 31.

--____ Periaortic
plexus- - - - - - - - - - - Notochord-------- Spinal cord

Section through the coelomic extension into umbilical cord.

Cross-sections of a 12-mm. Dolphin Embryo

Spinal cord.

Coelomic
extension
into umb. cord

Fig. 32.

Section through the caudal limb of intestinal loop (1).

55

56

H. HOSOKAWA

Spinal eord

Spinal ggl.

NotochordSympath. trunk

Aorta
R. subcard. vein

Mesonephros

Coelorn

R. umb. vein..,.,------ L. umb. vein----;~;..........:..._

Intestine (colon)

Coelomic_ _ _ _ _ extension
into umb. cord
Umbil. cord

R. umb. artery-----UrachusFig. 33.

· - - - - - - - - - L. umb. vein
Section through the caudal limb of intestinal loop (2).

Cross-sections of a 12-mm. Dolphin Embryo

57

58

H. HOSOKAWA

Tail----Post. card. vein._-- -----------

Somite ------~-----------

Notochord--------- Spinal cord

Fig. 35. Section through the root of umbilical cord (2).

~I

C
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fa1
2
‑mm. DolphinEmbryo

S
p
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.

Notochord

59

S
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Aorta

I
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Metanephros

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CaudalendofI.mesonephros

P
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Mesonephricduct

UrachusL.umb.arteryUrachusL.umb.v
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Fi
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60

H. HOSOKAWA

Spinal ganglion

//

',,

/ Spinal cord

,/'

',,

~//
',~ _/,~~}.~ ~,,,/ir ·:$~

Notochord -.....

-. . . . .
/ /J , ., ."1----~ /~// ~/
(_.&
.
~
•
.
cf
;~~--r~---1'-............

.

. ./

Caudfil ond Of ____r •. mesonephrosvein~--

0

~:i

0

/

/

"@

,-_-·Moranopfilooj~/ser?~d_1J
7
//

/

/

fov/',, ·'

0 ~

//
I
I
I

/

Urachus

I

I

I

o/ff'

I

\

'-,

\

/
~\
_ ,I
\\
I
I
I
\
\

I
I
I

I
I
I

'-..,
' , Mesonephric duct(=Wolffian duct)\\\

L. umbil. artery

Colon
Fig. 37. Section through the metanephroif(2).

11,

Caolom~~~ )f~-Z1; --------Mesocolon

\

/

/

_,Aorta~_,_- /

R. umb. artery ':9(.

Spino! nom-------

Post, card.

//

Root of/ intersegmental artery61

Cross-sections of a 12-mm: Dolphin Embryo

Spinal ggl.

Notochord -...,

Caudal end ofr. metanephros ...............

.........

.........

....................

.... ,------ L. post. card. vein-------~ Mesocolon-------- L. umb. arter~
'\

\

I

R. umb.

I

I/
I

Urachus

I

I
I

I
I

I
I

\

I

Genital tubercle
Fig. 38.

\\

\

\

\

' '\
' ' '\\\

I-------Hind limb bud''' ' ''\' \ Mesonephric duet(=Wolffian duct)

I

\

Colon

Section through the ureter, genital tubercle and hind limb bud.

H. HOSOKAWA

62

Fig. 39.

Section through the primitive bladder.

Cross-sections of a 12-mm. Dolphin Embryo

63

Spinal ggl.

Sympath- trunk

Notochord

L. post.
card. vein

L. ureter

Coelom

L .. umb.
arteryMesocolon
CoelomColon
Wolffian ductgoing into
. urogenital sinus

Hind limb bud

Urogenital sinus(primitive bladder)
Urogenital mtimbrane

Fig. 40.

Genital tubercle

Cloaca! septum

L. ureter going intourogenital sinus

Section through the urogenital sinus and membrane.

64

H. HOSOKAWA

Notochord - -----/ Sympath. trunk,,.,,,.,,,..,,.,.,,,,..
,,/

----

--Intersegmental artery-----

R. umb. artery----- ---L. ureter,_----Rectum
Hind limb bud------

Cloaca] septum - - - ---------- ---Urogenital sinus------- ----------

Tail-----------._--- - - Coelom---- Urogenital membrane,r---.,- Caudal'/

artery/////---- ------·Intersegmental
artery___ Spinal nerve

Notq<;hord- __---------

Somitl}S-----=-.::::~--

Spinal cord----~-_/--- Sclerotome---------- Myotome,/

Fig. 41. Section through the bifurcation of aorta into umbilical arteries.

'-,

'-,
\)

",,~

Cl

,/

/

/-

.........

' '·

.

P·

•••

//

Fig. 42.

/

/

//

0

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/

/;

......

~ [}J

/

__

--\-I

I

I

I

> •.

''

Vein "'°mp•nyi"g' '- ',' '-'-, Urogenital sinus(cloaca)----------Tail gut

w111it.c=== :~:,:· !~:,:'~,,•.RS. . . . •.· t\~~"•'ti

Demee em1d'l h•lf~~~""------ Spinal cord•1<} .. . • \;'~'if\···:'I,:>;;·'"';.:: ..

Fig. 43. Section through the tail gut.

Urogenital membrane......

Section through the caudal artery.

/> - -------- -- --

//

/

1
~/d~/j0-------~'LJ \)//Drogenital sinus(cloaca)

Rectum~""'""'

''

'.0)

~
\~

- ----....v_ . . . . . ,~ • ~l'-.
""'\)'-....,....,,---j '"-. .

'-Post. card. vein----Intersegment. vein

Caudal artery, -....,

Sympath. trunk,

-~
-~

I
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Spinal ggl. - - - - - - -

"-

n

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66

H. HOSOKAWA

Lateral processesof sclerotome

Myotomes ___ ...,.__------

Spina!

cord

Fig. 44.

Section through the lumbal curvature (1).

Cross-sections of a 12-mm. Dolphin Embryo

Lateralprocessesof sclerotome

Fig. 45.

Section through the lumbal curvature (2).

67

6
8

F
l
. HOSOKAW A

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PLATE

H. HOSOKAWA

Explanation of Plate I.
1. Three-layered wall of the neural tube. A Ependymal layer. B Mantle
layer. C Marginal layer. (Cf. explanation of Fig. 3)
2. Mitotic cells in the ependymal layer. The arrow points two mitotic cells
in the innermost zone of the ependymal layer of the neural tube.
3. Mitotic cells in the ependymal layer, which is cut tangentially. Sometwenty cells are found in various stages of mitosis.
4. Mitosis in the spinal ganglion. The arrow points a mitotic cell in theanaphase.

−S目 EBGHROH ・0ZJ

E注目 Hau
−

由己目−m
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嗣︒目︒出店

H. HOSOKAWA

Explanation of Plate II.
5. Transverse section of the Rathke's pouch. (Cf. explanation of Fig. 9) P
Posterior wall. A Anterior wall. I Infundibulum. Mitotic cells are encounteredin these epithelial walls.
6. Optic cup. (Figs. 9-11) I Inner layer or the primitive retina. 0 Outerlayer to furnish the pigment epithelium. The latter is provided in its inner zone
with many pigment granules, some of which are indicated by the arrow.
7. Primordial thymus. (Figs. 12-15) It arises from the ventral wings of thethird and fourth pharyngeal pouches. These cells of entodermal origin will proliferate and transform later into a syncytium that resembles reticular tissue, thus
forming the framework of the thymus.
8. Primitive thyroid gland. (Fig. 13) It is represented by a cell mass which
has originally invaginated from the ventral floor of the pharynx. Mitotic cells arescattered around. Several lumens inside the cell mass may be the remnants of the
thyroid diverticulum. A couple of weeks will elaspe before the follicular cavities
appear.

・ 2t ・
ω
5
0ZRUE− 目422ppg号 以 ・ロ
5E 2− P︿凶︒目︒出．出︿H. HOSOKAWA

Explanation of Plate III.
9. Lung bud. (Figs. 17-23) The bronchial bud shown in the center of the
figure is lined with ciliated, thich epithelium. When the bronchial buds continue
to grow and branch, the epithelial lining of the terminal portions become thincuboidal; and the alveolar primordia will occur in the later half of the felal life.
The mesothehal lining indicated by an arrow (P) represents the primitive pleura.
10. Liver. (Figs. 21-30) Larger cells with oval, coarse nuclei are the primitive
hepatic cells, some of which are in mitosis. Hepatic cell cords are separated fromeach other by the intervening sinusoids, a couple of which are encircled by brokenlines. The hepatic sinusoids are the site of hematopoiesis and are filled with various
kinds of primordial blood cells. The mesenchymal invasion to furnish the interstitial tissues will be prosperous later on.
11. Nucleated red cells in the circulating blood. In addition to the nucleatederythrocytes (E), there can be identified megaloblasts with larger nuclei (M) andnormoblasts with smaller, denser nuclei (N). The former is rather basophilic in thecytoplasm, while the latter is very rich in hemoglobin and stained deep red.
12. Primordial adrenal cortex. (Figs. 25-27) It is represented by mesenchymal
accumulation, which is encircled by broken line. The invasion of the primitive
medullary cells is not yet in view. (Cf. explanation of Fig. 25)

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Explanation of Plate IV.
13. Splenic primordium. (Figs. 26-27) It consists of the mesenchymal condensation just beneath the mesothelial lining of the dorsal mesogastrium. (Cf. explana·tion of Fig. 26)
14. Gonad in the genital ridge. (Figs. 25-34) The sexual gland is indifferent
or sexless at this stage, composed merely by inner cell mass covered by the germinal
epithelium (G. E.). Some mitotic cells are found in the latter.
15. Inner cell mass of the gonad. These cells have derived by the proliferative
ingrowth of the germinal epithelium. They are, however, not the progenitors of
the real germ cells. For, the primordial germ cells are thought to originate in theyolk-sac entoderm, migrating afterwards into the genitial ridges.
16. Primitive pancreas. (Figs. 28-29) The future pancreas is indicated by outpouchings from the duodenum into the dorsal mesentery. As they continue to grow,they become contorted, thus forming the primitive pancreatic ducts lined with entodermal epithelium (D). Later on, the acini begin to appear as terminal and side buds
from the ducts. Also the Langerhans' islands will differentiate from the same
source.

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Explanation of Plate V.
17. Cross section of the mesonephros. (Fig. 23-37) (Cf. explanation of Fig. 32)
G Glomernlus. S Secretory segment. C Collecting segment. W Meso~ephricor Wolffian duct. B Connection between the Bowman's capsule and the secretory
segment.
18. Metanephros. (Figs. 36-37) The tuba!, epithelial portion (C) is the primordial pelvis, calyces as well as of the collecting segments. The mesenchymal
accumulation (S) surrounding the tubules represents the primorclium of the secretory
segments. (Cf. explanation of Fig. 37)

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