AN EARLY DOLPHIN EMBRYO (STENELLA
COERULEOALBUS) IN SERIAL SECTIONSJOHN G. SINCLAIR*This 4.2 mm. dolphin embryo is probably the youngest to be completelydescribed. A 12 mm. embryo was described by H. Hosokawa in 1955
in complete serial sections i). Embryos of the humpback whale werestudied and described by Stump, Robins and Garde 2). The present
embryo was taken at Kawana on Izu peninsula Japan, Dec. 7, 1960.
The dolphin drive which takes place twice a year was rather lightyielding 300 animals whereas some drives reach 1000. Of dolphins killedin this drive approximately half were females. Several species wereincluded but Stenella predominated. Females were of all ages but about
twenty younger ones were examined and, using the corpora lutea asindicators, ten uteri were removed. Three of the ten contained blood
due to rough handling in killing and the remainder yielded only one
embryo. The others apparently had aborted.
This result together with the fact that only one embryo could be secured in a second drive a month later, from approximately fifty females
in a catch of 600 dolphins, leads one to think that the rate of reproduction in this cetacean is extremely low. Dr. T. Ogawa of Tokyo
University had insisted that young embryos could be secured only inearly December because the period of breeding is probably late Octoberor early November. The size of the present embryo indicates an ageof about one month and confirms his estimate. A second embryo of26 mm. taken Jan. 20, 1961 is about 2·1/2 months old and again confirms
his judgement of the breeding period. This data also fits the second ofthe breeding periods for the Fin Whale deduced from Internationalstatistics 3), According to Stump, Robins and Garde 2) the mating is ata maximum for the humpbacked whales in August in the eastern andwestern Australian waters.
The external features of the embryo are remarkable. Fig. 1. The
external appearance of a whale embryo of the same dimensions wasgiven by Dr. Nishiwaki in 1957 4 ), In our dolphin embryo the wholetrunk is a helix of one full turn about the body stalk as an axis.

* The author is in Japan under Research Grant NONR-3355(00) between the Office ofNaval Research and the University of Texas. Dr. M. Nishiwaki of the Whales Research Institute, Tokyo, secured the embryo. The Department of Anatomy, Tokyo University Faculty of Medicine provided the facilities for preparation of sections and photography. I am
endebted to both institutions.
83

84

JOHN G. SINCLAIR

Various degrees of spiralling may be seen in many mammal embryos
and the author has two human embryos of the same dimensions andequal amount of torsion.
The embryo was removed from a gestation sac 65 mm. in length.
Fig. 2. The placenta is extremely simple like that of the pig andseparated from the uterus under a small stream of water. A detail ofcorpus luteum of the maternal ovary is seen in Fig. 3. Its dimensions
were 32 x 36 mm. The gross cut surface of this corpus luteum is homogeneous. Those taken from uteri suspected of recent abortion showeda more cordlike structure.
These serial sections were cut from celloidin at 10 microns and werestained with hematoxylin and eosin. Standard orientation presents themidbrain first. Section 14, Fig. 4, shows the germinal zone of the brain
and surrounding marginal zone. No mantle zone is discernable. By
section 20, Fig. 5, the narrow cleft opening into the third ventricle andexpansion of the fourth ventricle are distinguishable. The roof of thehindbrain (metencephalon) is already folded although it is thick. Small
extensions of the cerebral plexus foreshadow the carotid and cardinal
systems. Section 31, Fig. 6, shows the forbrain and hindbrain separated
by the cephalic flexure. Neural crest may be distinguished in the region
which becomes the trigeminal ganglion. Note the skin depression andits adherence to the neural mass. An interesting detail Fig. 7, is fromthe angle between forebrain and optic vesicle. Here may be seen a
stream of basophil cells still attached at the angle and stretching down
into the mandibular arch. They are said to take an important part in
the structure of that arch.
In section 38 Fig. 8 the optic vesicles are seen to be narrow extensions of the forebrain barely contacting the epidermis. There is no
lenticular thickening and no terminal expansion of the vesicle. Basophilcells are seen both cranial and caudal to it. The infundibulum of thethird ventricle does not contact the notochord any longer. The latter
is fused to the roof of the preoral gut. The hindbrain has developed
neuromeres between the potential origins of fifth and seventh nerves.
Note that the seventh ganglionic aggregate also contacts the epidermis
closely.
By section 43 Fig. 9 we reach the middle of the open octocyst. Asymmetry due to torsion begins to appear. By section 52 Fig. 10 theotocysts are cut ventrally and the ganglionic primordia of nerves VIIIIX are indicated. The anterior cardinal vein becomes a prominentlandmark. First and second aortic arches and their dorsal connections
are present. The preoral gut (Seessel's pouch) between the first arches
ends on the pharyngeal membrane. Cranial to that are two diverticulaAN EARLY DOLPHIN EMBRYO

85

of Rathke's stomodeal pouch lateral to the infundibulum. The anterior
neuropore is still open over the telencephalon.
By section 57 Fig. 11 the stomodeum opens upon the left side and theunruptured membrane is suspended between mandibular arches. Firstand second arches are seen in cross section together with their associatedfifth and seventh ganglionic primordia. The first pouch reaches its closing
plate and the first furrow does the same. The last trace of otocyst isseen. Any occipital somites between this and the first cervical are
already fused into one mass. Note the small size of the brain and thethin terminal lamina. The tip of the head is in section 65 Fig. 12.
The second branchial furrow is on the left together with an unexplained
endodermal microvesicle. The first somite to show a definite dermamyotome is present. Ganglion X is indicated.
Section 78 Fig. 13 shows the margin of the second so mite on the left
and the cervical opercular bulge candal to three well defined arches ofnearly equal size. The descending aorta is present. The spinal cordshows no differentiation of mantle layer and there is no evidence ofspinal ganglia. Thick endodermal epithelium indicates future developments. Most of the medial surface of the first arch is ectodemal reaching back to the deep phayngeal membrane. By section 83 we see thefloor of the pharynx and note the ventral aorta reaching the first arch.
Second and third pouches reach closing plates without perforation.
Somite three begins to show on the left.
By section 96 Fig. 14 the bulbus arteriosus shows at the base of thesecond arch and a section of truncus arteriosus lies in the pericardial
cavity. Mandibular arches have not yet closed into a symphysis. Thepharynx is broad at the fourth pouches. Descending aortae lie justventral to the third somite. The spinal cord is thick and the lumen
narrow. It shows no distinction into basal and alar plates. By section
102 Fig. 15 the pharynx narrows and the thickened pulmonary ridges
develop just lateral to the thick endoderm foreshadowing lung buds.
The truncus arteriosus and part of the ventricle show a suspended
endocardial tube and many extravascular erythroblasts.
In section 106 Fig. 16 the ventricular loop gives way to the central
atrium. Pulmonary ridges are prominent but the pharynx narrows.
Neither glottis nor trachea is present. Somite four appears on the left.
Lateral mesocardia containing common cardinal veins appear in section
120 and are more symmetrically developed in section 126 Fig. 17. This
results in forming a pair of pleuroperitoneal canals. Between thesejust ventral to the gut are the cordlike diverticula of the liver andgall bladder. The latter does not get beyond a rudimentary stage.
Pancreatic diverticula also are present but difficult to demonstrate.
1

86

JOHN G. SINCLAIR

Somite five is present. Still in the segment of five in section 142 Fig.
18 the liver cords extend profusely and lateral diverticula of the duodenum are evident. The face of the liver lining the pericardia! cavity
is the septum transversum. Liver extends to the lateral walls andincorporates vitelline and allantoic veins into hepatic sinusoids.
Section 159 shows the intestinal portal as the gut opens up opposite
somite six and the liver is separated into lobes containing paired vitelline or omphalomesenteric veins. The epithelial and vascular structureof the yolk sac wall are well displayed in folded membranes. Blood
making functions have already been largely transferred to the generalcirculation. In section 168 Fig. 19 the right vitelline vein passes upward toward the liver. Heavy parietal walls indicate the future limb
buds.
In section 174 the coelom opens laterally to the extraembryonic coelom.
At somite seven there is the first appearance of mesonephric duct
cranial to any tubules. Limb primordia are still just parietal thickenings.
Section 186 Fig. 20. The spiral torsion of the trunk is evident insomites 7-8. The coelom on both sides is open. Aortae remain double.
Mesonephric tubules appear in the nephric ridge. The gut is again
closed by the folded yolk sac. By section 193 the whole border of theloop is cut in a half circle. The wall of the yolk sac is central to theloop. Ten mesonephric tubules appear. By section 198 Fig. 21 a ninth
somite is added and the coelom is seen extending to the caudal end.
Section 207 Fig. 22 cuts the full line of somites from nine throughtwenty-nine. The last somite ends in the unsegemental lateral mesodermal plate. The whole remaining mesonephros is displayed with its duct.
There are approximately 2·1/2 tubules per somite but neither glomeruli
nor cloaca! connection to permit function.
Shifting caudally to section 220 Fig. 23 the spiral is cut through theopen posterior neuropore. Dermatome, myotome and sclerotome of eachsomite are distinct and the dorsal intersomitic invasion by somatopleureis in process.
By section 223 Fig. 24 the notochord terminates in the primitive knot
and the cloaca! plate is indicated. The superior mesenteric artery descends and the gut is defined back to the rectum. Section 235 Fig. 25
shows the cloaca! expansion to the anal plate and proctodeum on theone hand and the dorsal wing leading to the mesonephric duct on theother. These features are further expanded in section 248 Fig. 26.
This sectiton passes through the urachus (allantoic duct) into the thickwalled allantois. Note that at this stage it is much less vascular thanthe yolk sac. The urethral plate is better shown in section 251. FinallyAN EARLY DOLPHIN EMBRYO

87

section 275 Fig. 27 shows the expanding end of the allantois extending
caudally through the gut loop.

REFERENCES
1)
2)

3)

4)

HOSOKWA H. (1955). Cross sections of a 12 mm dolphin embryo. Scientific Reports
Whales Research Institute, 10; 1-73.
STUMP C. W., J.P. ROBINS and M. L. GARDE (1960). The development of the embryoand membranes of the humpback whale, megaptera nodosa (Bonnaterre). Austr. J.
Marine and Freshwater Research 11; 365-368.
NAKTGEBORNEN c., E. J. SLIJPER and w. L. VAN UTRECHT (1960). Remarks on theperiod of conception, duration of gestation and growth of the fetus of the fin whalebased on data from International Whale Statistics. Norsk. Hvalfangst Tidende, 1960
# 3; 313-319.
NISHIWAKI M. (1957). Very smal !embryo of cetacea. Scient. Rep. Whales Res. Institute 12; 191-2.

.SINCLAIR
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PLATEII

JOHNG.SINCLAIR‑. Fig. 4.
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Fig. ．
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forebrain and opticvesicle.

Sei.Rep. Whales.Res.Inst. No.1
6

JOHNG.SINCLAIR

PLATEIIIFig. 8.
Section )8.

Fig. 9.
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JSei.Rep. Whales.R
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JOHNG. SINCLAIR

PLATEIV

Fig. 12.
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Fig. 13.
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ら告Sei.Rep.W
hales.Res.Inst.No.1
6

JOHN G. SINCLAIR

PLATE V

Fig. 16. (Section 106)Fig, 17. (Section 126)

4s.
s. 4
Pharynx
Phar.trach.
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Ventricle
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veins_- Septum
transv.
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f;:~.:;917;trff5~~:"'!J:-----_Endocardium___ .,.-.~Parietal

pericardialwallFig. 19. (Section 168)
Fig. 18. (Section 142)
Somi te 6
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canalSomite 4Somi te 5Nephrogen. iridge'Stomach'Lateralumbil.

Omphalo·mesent. v.

~~:-7'~-'+#!!.'--- Pericard,

cavitytSei. Rep. Whales. Res. Inst. No. 16

JOHNG.SINCLAIR

PLATEVl

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Fig. 21・（ Section198)
Somite 8
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PLATEVIIJOHN G.SI
NCLAIR

Fig. 22. (Section 207)

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，、{Fig. 2う. (Section 220)
Posterior neuropore
Tail bud
Spinal cord

Ne~hricridge
Coelo皿
Aorta

NotochordNeura:). canal.Somite 23

Sei.Rep.W
hales.Res.bもst.No.1
6

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PLATEIX

JOHNG. SINCLAIR

Fig. 26. (Section 248)
CloacaUrachus
Coelom

1Fig. 2
7. (Section 275)

Allantoic
cavityPoorlyvascular wall

ドーSei.Rep.Whales.Res.Inst.No.16