PRENATAL DEVELOPMENT OF EAR PLUG
IN BALEEN WHALESTADAYOSHI ICHIHARA
The ear plug is a fairly soft tissue existing in the external auditory meatus of the
Mysticeti. It is composed of independent two tissues derived from the epidermisof external· auditory meatus and from that of so-called glove-finger. The glovefinger projects outwards from the tympanic bulla into the external auditory meatus.
Since Purves (1955) found out ring-like laminae in the longitudinally bisected plugcore derived from the epidermis of the glove-finger, the plug-core has encharmed
many cetacean biologists interested in determining the age of whales. Counting
the ring-like laminae to evaluate age of baleen whales is no more than understanding life histories of these whales through their ear plugs.
As series of studies on ear plugs, I have much interested in the formation of ear
plugs in the prenatal stage, because animal tissues begin to develop from the prenatal
period and the epidermis of the glove-finger also belongs to the same growth category.
It was also necessary to confirm my previous finding (Ichihara, 1959) through themany materials from the baleen whale foetuses. In this paper, I examine the morphology and histology for the prenatal development of the glove-finger along withthe ear plug. The keratinized and fatty degeneration of epidermal cells of the
glove-finger are alternately accumulated to form the plug-core in the postnatal stageof the baleen whale. The present study not only supports this fundamental mechanism found by me, but also gives some suggestions to interpret the factors affecting the formation of the plug-core.
I am much indebted to Dr. Hideo Omura, the Director of our Institute for hiskind support throughout this work. My sincere thanks are dut to Messrs. Toshitomo Shida and Yasuji Fukuda, the Department of Anatomy, Faculty of Medicine,
University of Tokyo, for their kind helps in the histological examination and photomicrographic technique. I also indebted to Dr. Kazuo Fujino, a staff of our Institute, who collected several heads from fin whale foetuses in the Antarctic expedition and brought them to my study.
MATERIAL AND METHOD

Most of materials in this paper were collected by myself on board Nisshin-maru,
a factoryship of Taiyo Gyogyo Company in the Antarctic expedition in the 1959/60
season. Foetuses were taken from the uteri of £lensed mother whales. After measuring the lengths of foetuses, their heads were cut off and pieces available for this study,
that is, posterior portions of heads including the external auditory meatus, glovefinger and tympanic bulla were obtained. These specimens were preserved in the
10% formalin solution soon after this treatment. The large blocks from full-grownfoetuses are preserved in diluted formalin solution under 10% as the first step and29

30

T. ICHIHARA

removed gradually into concentrated solution for the examination at our Institute.
In the same way Dr. Fujino kindly presented me several heads from fin whale foetuses in the Kyokuyo-maru No. 2 expedition in the 1960/61 Antarctic season. These
specimens are used to check my interpretation. The examined specimens which I
collected comprise 48 Antarctic fin whale (Balaenoptera physalus), 17 sei whale (Balaenoptera borealis) and 2 humpback whale (Megaptera novaeangliae) foetuses as presented
in the appendix.
Law's finding (1959) for the foetal growth curve is applied to determining
accurate foetal months from the lengths of examined foetuses. According to Laws
the average duration of gestation is 11 1/4 months for the Antarctic fin whale andabout 12 months for the North Pacific sei whale.
I counted numbers of ovulation in mother's ovaries at the spot aboard the factory
ship to evaluate the relative age of the mother whale. The examined foetuses are
obtained from mother whales of various ages and there is no bias in the sampling,in which I intended to obtain the foetuses of various length. The specimens from
foetuses under 71 cm, corresponding to the 3 3/4 foetal months, also were obtained
but failed to be examined in the sectioning course. These blocks from heads
were dissected for the morphological study. Later glove-fingers and ear plugs wereembedded in the celloidin solution, and sectioned in the thickness from 10 to 15 µ.
The staining method by haematoxylin eosin was applied.
The post-mortem times range from 1 hour and 25 minutes to 13 hours and 35
minutes for the foetuses of fin whales, from 45 minutes to 9 hours and 40 minutes
for the sei whale foetuses, and from 3 hours and 45 minutes to 7 hours and 5 minutes
for the humpback whale foetuses, as listed in the appendix. In addition to thespecimens from such foetuses, I examined the prenatal layers of ear plugs fromadult baleen whales, which also includes the blue (Balaenoptera musculus), theBryde's (Balanoptera edeni) and the minke whale (Balaenoptera acutorostrata).
MORPHOLOGY OF GLOVE-FINGER AND EAR PLUG
In the course of dissecting the external auditory meatus of foetuses from the earhole to the tympanic bulla through the blubber, I observed the colour on the surfaceof auditory meatus and glove-finger and then measured the length of space fromthe blind section beneath the blubber to the distal end of the glove-finger. Taking
the glove-finger out of the auditory meatus, I measured the length of glove-finger,from its distal end to the proximal end of tympanic ligament, and its breadth.
From the colour on the surface of glove-finger and from that on the internal
wall of external auditory meatus, four stages are present in the foetal stage of thebaleen whale as indicated in Table 1. In fin whales, stage I indicates the development until the 3rd and 4th foetal months, stage II the 5th and 6th foetal months,
stage III from the 7th to 9th months and stage IV after the IOth foetal months.
In sei whles, stage II represents the 5th to 7th foetal months. Glove-fingers of
foetuses in stage I are transparent and rather whiter. From stage II to III theircolours change from pinkish-white to dark-brown. In stage IV, glove-fingers of

PRENATAL EAR PLUG

31

fin whales are brown, while those of sei whales do gradually black. Ear plugs fromadult sei whales is more black than that from [fin whales. The content of melanin
during the keratinized degeneration of epidermal cells probably is more abundant
for the sei whale than for the fin whale. The colours of stage IV are maintained
from juvenile to adult both for fin whales and for sei whales. The observation for
foetuses of the humpback whale is insufficient for describing the colour change during foetal development but glove-fingers are pinkish-white in stage II.
TABLE 1.

FIN WHALE
Pregnancy
Stage
month

COLOUR OF GLOVE-FINGER, EXTERNAL AUDITORY MEATUS
AND BLIND SECTION FOR EACH FOETAL MONTH
OF THREE SPECIES IN THE BALEEN WHALE

External auditory meatus

Glove-finger

I {!

White (transparency)~

Pinkish-white

II {

~

Light-brown
Dark-brown

Light-brown, Black antero-dorsal portion
Dark-brown"
Dark-brown

Dark-brown

III {

IV

10

"

White
Pinkish-white"

Blind section

Yellw
Yellowish-white"

SEI WHALE
Stage

Pregnancy
month
5

II {
III {

IV

Glove-finger
Pinkish-whiteLight-brown
Dark-brown
Black"
Light-brown
Dark-brown
Black

6
7
8

9
10

External auditory meatus
Pinkish-white"

Blind section

Black antero-dorsal portionHUMPBACK WHALE
Stage
II

Pregnancy
month

Glove-finger

6

Pinkish-white

External auditory meatus
Pinkish-white,

Blind section

Black antero-dorsal portion

The colour on the internal wall of external auditory meatus is very similar tothat on the surface of the glove-finger but it becomes black at stage III particularlyin the antero-dorsal portion. The black portion tends to extend gradually to otherpart of the external auditory meatus. The black part occurs in stage II for sei andhumpback whale foetuses.
In the early foetal month, the colour on the suface of glove-finger depends on
the thickness of connective tissue and the abundance of blood capillaries or bloodvessels. However the brown colour over stage III indicates the occurrence ofadhesive materials on the surface of the glove-finger and of the internal wall of theexternal auditory meatus. In fact, the original colour of the glove-finger and theexternal auditory meatus appears again after scraping these materials off. From

T. ICHIHARA

32
12
10

A. length of tympanic

8
6
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2

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u

10

B. Length of glove-finger

8
6
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I

2

•
•

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5

~

6

•
•

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11Pregnancy Month

Fig. I. Growth of tympanic bulla, glove-finger and external auditory meatus in the fin
whale foetus from the Antarctic.
12
lO

A Length of tympanic bull a

8
E 6

4

u

2
l0

..

B. Length of glove-finger

8
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E 2
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~ 30
~20

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Distal end of glove-finger
to blind section

. ..

10

.

2

3

4

5

6

7

8

9

l0

...
11

Pregnancy Month

Fig. 2. Growth of tympanic bulla, glove-finger and external auditory meatus in the sei
whale foetus from the Antarctic.

12

PRENATAL EAR PLUG

33

the naked-eye observation, the horny layer of foetal epidermis seems to be accumulated
from 7th foetal month for fin whales and from about the 8th foetal month for sei
whales. In fin foetuses, the yellow adhesive materials are observed on the blind
section of the auditory meatus over the 8th foetal month. These materials develop
into ear plugs and observed by the naked-eye as various shapes in the latter half of
the 9th foetal months.
To examine the transformation of the glove-finger, its length and greatest
breadth were measured. The measurement of the distance from the distal endof the glove-finger to the blind section of the external auditory meatus wasneeded for examining the mechanism which the prenatal ear plug is preserved
throughout the life span of whale. Fig. I indicates the relation between the development of the glove-finger and that of the external auditory meatus. The shapeof glove-finger changes from the early to the late foetal stage. The length of glovefinger from its distal end to the end of tympanic ligament increases rapidly after 4th
or 5th foetal months for the fin and sei whale. Until the middle foetal stage, thegreatest breadth is indefinitely measured but estimated to grow in the same way as
the increment oflength. As shown in Fig. 1, the length of tympanic bulla, which
is a bony tissue closely connected with the glove-finger, also increases in a sigmoid
curve in the foetal stage. The size of the tympanic bulla seems to increase slightly
in the postnatal stage, while the growth of the glove-finger is completed in the latefoetal stage. Although the size of the sei whale golve-finger is different from that of
the fin whale, the growth pattern is about the same between two species (Fig. 2).
This observation may be applied to the other baleen whales.
The relation between the glove-finger and the external auditory meatus is of
most importance to pursue the prenatal development of the ear plug. In fin whale
foetuses the space between the blind section of the external auditory meatus and theglove-finger is large in the early stage. At each foetal month, the relative value ofthe space changes as indicated in the lowest in Figs. 1 & 2. The space in thesefigures represents the length from the distal end of the glove-finger to the blind section of the external auditory meatus. If the relative value of the space is expressed
as a percentage of the length of the auditory meatus from the proximal end to theblind section, it distributes along the concave curve with the developing foetal month.
The external auditory meatus is closely connected with the glove-finger at its proximal
end. In fin whales the relative space exceeds 40 percent under the 4th foetal monthand approaches to zero percent from the 8th to 9th foetal months, when some foetuseshave no space between the blind section and the glove-finber. In the late foetal
month, it increases again and is supposed to extend rapidly at the postnatal stage.
In fact the external auditory meatus of the adult whale is elongated along thesquamosal region of the large skull. This change of space also is drawn in Fig. 3.
On the other hand the sei whale foetus has a fairly larger space in the 8th,
9th and lOth foetal months, compared with the fin whale foetus. The space is 10
percent in these foetal stages, as shown in the lowest of Fig. 2 and there is only one
foetus having no space. The change of space in Fig. 1 & 2 represents an evidence
that the development of the external auditory meatus is not proportionate to that

T. ICHIHARA

34

of the glove-finger in the foetal stage. In two humpback whale foetuses, relativevalues of spaces are 29.6 and 44.4 percent but these foetuses are in the 6th foetal
months. This evidence probably is expanded for the other species of the balaenopterid whale than fin, sei and humpback whales.
BS

i

BS

BS

GF

2

3

Fig. 3. Relation between the gave-finger and the external auditory meatus in the developing foetus of the fin whale.
1. The 4th foetal month
BS :Blind section
EAM: External auditory meatus
2. The 7th foetal month
3. The lOth foetal month GF:
Glove-finger

In fin whale foetuses the external auditory meatus becomes to cover the outerside of the glove-finger after the keratinized epidermal cells was accumulated. Therefore, the prenatal plug-core of the fin whale is surrounded by the keratinized layer
derived from the epithelium of the external auditory meatus. The mechanism thatthe prenatal ear plug is preserved for the all life of fin whales is interpreted from thisfinding. In most of sei whale foetuses, the external auditory meatus does notsurround the surface of the glove-finger throughout the foetal stage. The prenatal
ear plug, therefore, is unstably accumulated. In fact, it is very difficult to findthe prenatal layer in the ear plug from the adult sei whale. At the distal end of
the plug-core, it is easily separated from the outer covering. The practical collectionof whole ear plugs to evaluate the age is difficult for the sei whale.

35

PRENATAL EAR PLUG

From the presence of the prenatal ear plug and space of the external auditory
meatus, baleen whales are divided into two categories. The one includes the ear
plugs from blue, fin, Bryde's and humpback whales. In the most of these whales,prenatal ear plugs are preserved completely throughout the life span. The othercontains the ear plugs from sei and minke whales, in which prenatal layers are indistinguishable. In the former category, the shape of prenatal ear plug depends upon
the shape of space between the glove-finger and the internal wall of the external
auditory meatus near the full foetal month. I draw the shape of the prenatal ear
plug of the fin whale in Fig. 4. The ear plugs accumulated at the distal end of the
glove-finger develops according to the shape of the increasing space after the 8th
foetal months.
BS
BS

BS

GF

GF

GF

Fig. 4. Shapes of ear plug in the late foetal stage of the fin whale. It correlates with theshape of space from the distal end of glove-finger to the blind section of external auditory
meatus.
BS : Blind section of external auditory meatus.
EP: Ear plug.
GF: Glover-finger.

There are various kinds of shapes in the prenatal ear plugs. The most dominant shape is crescent and the triangular shape sometimes present. Such a finding
is applied to blue, Bryde's and humpback whale foetuses. In the ear plugs fromadult blue, Bryde's and humpback whales there remain various shapes of prenatal
layers.
HISTOLOGY

The study on the formation of prenatal ear plug should be approached by the histological examination. When age of baleen whales are estimated by ear plugs,
the core part is more important than the outer-covering (Purves, 1955), although thecore and outer-covering constitute socalled ear plugs of baleen whales. The histological examination in this paper, therefore, was forcused on the plug-core derived
from the degenerated epidermal cells of the glove-finger. Ichihara (1959) points
out that the celloidin section suits for the hard materials like the glove-finger andthe frozen section for the soft materials like the ear plug. In the foetal stage, how-36

T. ICHIHARA

3200
Antarctic Fin Whale

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2800

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Pregnancy

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Month

Fig. 5. The increasing thickness of the connective tissue of the glove-finger from the fin
whale foetus. The broken line is estimated.

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Pregnancy Month

Fig. 6. The increasing thickness of the connective tissue of the glove-finger from the sei
whale foetus. The broken line is estimated.

PRENATAL EAR PLUG

37

ever, very thin ear plugs are accumulated on the surface of the glove-finger, andhence I used only the celloidin section for this study. On the basis of the sectionof thickness from 10 to 15 µ, I measured the thickness of the connective tissue ofthe glove-finger, that of the keratinized layer and fatty degenerated layer of the
glove-finger epidermis for each specimen. My observations also are made forthe degeneration of nucleus of the germinal layers and for the occurrence of thekeratinized layer densely stained by eosin.
Thickness of connective tissue: The thickness of the connective tissue was measured
between the basal layer of the outer epidermis and that of the inner one of the glovefinger. As papillary layers undulate slightly after the 6th foetal months, the meanvalue is obtained from several measurements for each specimen. The thickness ofthe connective tissue is plotted against each foetal month of the fin whale as shownin Fig. 5. The broken line is estimated in the figure, because available data are
insufficient. There is a suitable straight line for the increasing thickness until about
the 6th foetal month and then the thickness grows rapidly until the full foetal month.
Just after birth, the thickness of the connective tissue approaches to about 2900µ,
the mean value for the adult fin whale.
The thickness of the connective tissue represents the same increment for the sei
whale foetus but it is thinner than for the fin whale foetus as, indicated in Fig. 6.
Keratinization: A specimen from the 3 3/4 to 4th foetal months is covered with
two layered epidermis. The outer layer, the periderm, constitutes a protective
covering although the nucleus is in a depressed form (see Plate I, Fig. 1). The basal
layer of early embryo is composed of low cuboidal cells with clearly stainable nuclei
and a smooth basal surface. With the increasing embryo, the basal cells become
columnar with large, darkly stained nuclei and proliferative. Even in a specimen
from the 3 3/4 to 4th months, the compact basal layer is covered by periderm cells,
some of which are exfoliating.
In the human embryo, a stratum intermedium, proliferated from the basal layer
develops between the basal layer and periderm at about the 4th foetal months andtoward the end of foetal life it, now called stratum spinosum, or prickle cell layer,
is several cells thick (Montagna, 1962). These processes also exist in the epitheliumof baleen whale foetuses.
The periderm of the foetal glove-finger is stained lightly by eosin. Montagna(1962) states that the keratohyalin granules appear where the epidermis is thickest
but the general epidermis has no stratum granulosum at the end of the 5th months
in the human embryo and that the keratinization progresses only when the stratum
granulosum is formed. The stratum granulosum is not observed definitely in the
thick epidermis of the glove-finger from the baleen whale, although it is assumed
that the same mechanism as in the man progresses in the keratinization of thewhale epidermis.
To examine when the keratinization begins, I measured the thickness from theupper edge of the basal layer to the most distal end of accumulated substance foreach specimen from fin whale foetuses and plotted it against each foetal month in
Fig. 7. The thickness expressed in such a way certainly includes both the kera-

38

T. ICHIHARA

220
200

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Fig. 7. The increasing thickness of the accumulated layer of the fin whale foetus, including the keratinization and the fatty degeneration layer in the epidermis of the glovefinger. Broken line is estimated and chain line indicates the mean curve.

::,c"'0 40

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Fig. 8. The increasing thickness of darkly stained part by eosin with the advancement of
the foetal month of the fin whale. This part is present at the lowest of keratinizedlayer.

PRENATAL EAR PLUG

39

tinized layer and the fatty layer described later. Therefore, the thickness of keratinized layer will be obtained by subtracting the fatty layer from total accumulated
layer. It is difficult to examine accurately the primitively keratinized cell from the
prickle cell layer in the late foetal stage.
Fig. 7 indicates the accumulation speed in the fin whale foetus. From fertilization to the 6 l /2 foetal months, the accumulation progresses in a constant ratio,
although it is estimated by the broken line in the early foetal stage from fertilization
through the 3rd foetal month. After the 6 l /2 foetal months, the accumulated layer
is rapidly thick but there are great variations for individual specimens. An accumulated layer from one specimen of the 8 1/4-8 2/4 foetal months is 119 µthick,while that from another specimen of the same foetal month is 28 µ thick. As theexternal auditory meatus covers the surface of the glove-finger after the 8th foetal
months, it is difficult to examine the whole accumulated layer only from sections ofglove-fingers. The prenatal ear plug, disconnected from the surface of glove-finger,
indicates various heights. The values of the keratinized layers are about 4300 11
for a specimen from the 9 2/4-3/4 foetal months, about 850 µfor that from the 1010 1/4 foetal months and about 3800 µfor that from the 10 3/4-11 foetal months.
These variations probably depends on the activity of the mitotic division on
the prickle-cell layer or the basal layer of the epidermis. There are also great
variations for the lengths of plug-cores from adult fin whales (Nishiwaki, Ichihara & Ohsumi, 1958: Purves & Mountford, 1959).
The germinal layer also becomes thick rapidly after the 6th foetal months andis maintained for the late foetal month. Probably the thickness of the germinal
layer and hence the number in stratified cells, also will be kept in the postnatal life.
At the same, the layer stained darkly by eosin begins to occur in the distal endof the accumulated layer as indicated in Plate II, Fig. 1 & III, Fig. I. This darkly
stained layer is thick with the growing foetuses and reaches about 40 µ in thickness
at birth. The increment of this layer for examined specimens is plotted against eachfoetal month of the fin whale in Fig. 8. Although the periderm in the early foetal stage
appears to undergo cornification, it probably does not mean the keratinization.
The occurrence of darkly stained layer seems to indicate the keratinization in theepidermis of the glove-finger.
In the sei whale foetus as well as in the fin whale foetus, the darkly stained
layer occurs when the accumulation ratio increases rapidly. Fig. 9 shows the increasing thickness of total accumulated layer against each foetal month of the sei
whale, although available specimens are insufficient. The keratinization in the epidermis of the glove-finger arises at the 8 l /2 foetal months of the Antarctic sei whale.
For the sei whale foetus, there is a divergence between the growth curve by
obtained by Matthews (1938) and that indicated by Laws (1959). Interpretationby Matthews results from the Antarctic sei whale, while Laws has his conclusion on
the basis of the North Pacific sei whale, therefore, the divergence may be derived
from the localities of sei whales. In determining the foetal month of the sei whale,
I adopted Laws' finding, because his curve fits the growth patterns of otherbalaenopterid whales. I am afraid, however, the keratinization for the Antarctic

T. ICHIHARA

40

sei whale foetus occurs slightly earlier than the 8th foetal month. There are also
great variations in the thickness of the keratinized layer for the individual foetusof the sei whale. In the 10 1/2 foetal months, an accumulated layer from a
specimen is 273 µ thick, while that from another specimen is 35 µ thick.
The keratinized layer in the foetal stage remains in the ear plug of the adultbaleen whale. Plate V indicates the presence of the prenatal keratinized layer,
appearing at the most distal end of the plug-core, outside of the prenatal fatty layer
for various baleen whales. The prenatal keratinized layer is observed more clearlyin the fin, blue, Bryde's and humpback whale than in the sei and minke whale.

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Pregnancy Month

Fig. 9. The increasing thickness of the accumulated layer of the sei whale foetus, including
the keratinizatin and the fatty degeneration layer in the epidermis of the glove-finger.
Broken line is estimated and chain line indicates the mean curve.

Fatty degeneration: The germinal layer becomes thick with the advancementof the foetal month and composed of cells in several layer at the 6 1/2 foetal months.
The outside prickle-cell layer continue to degenerate into the keratinized cells
from the 6 l /2 to the 8th foetal months, when slightly swelled cells in the pricklecell layer sometimes contain lipoid granules in their cystopasms. This indicatesthe occurrence of fatty degenerated cells in the epithelium of the glove-finger. The
fatty degeneration progresses rapidly in the whole prickle-cell layer in larger foetuses
after the 8th foetal months. The cystoplasms filled with lipoid granules develop
into the fatty layer which is removed outwards by the pressure of newly degenerated
fatty cells. The accumulation processes of fatty layers are clearly indicated in Plate
III, Fig. 1 & 2 and IV, Fig. 1 & 2. From the lOth to 11 th foetal months of the
fin whale, the fatty layer is from 33 µ to 130 µ thick and estimated rapidly thickat birth.
As the fatty layer remains on the surface of glove-finger, its thickness can bemeasured accurately through the section. Fig. 10 indicates the increment of fatty
layer against each foetal month of fin whales. The thickness of the fatty layer ismeasured between the lower border of the keratinized layer and the basal layerof the glove-finger epidermis. According to this figure, the fatty degenerated cells•

PRENATAL EAR PLUG

41

appears at the 8th foetal months and the fatty layer develops over 180 µ in thicknessfor 3 l /4 month until the birth of the fin whale foetus. Through the microscopic
examination, my previous finding (1959) can be applied to the process of fatty
degeneration in the prenal ear plug. This suggests that the fatty degeneration passes
through the same mechanism in the prenatal as well as in the postnatal ear plugs.
In specimens from sei whale foetuses, fatty granules are found for the first timein the cystoplasm of degenerated cells in the 10 1/4 foetal months. Although a fe\vspecimens do not represent the rigid finding, it is assumed that sei whale foetuses
develop more thin fatty layer than do fin whale foetuses.

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Fjg. 10. The increasing thickness of the prenatal fatty layer of the fin whale, composedof fatty degenerated cells in the epidermis of the glove-finger.

By the naked eye observation, these fatty layers remain at the distal end of all
ear plugs from fin, blue, Bryde's and humpback whales, therefore, it can be recordedby the photometric method (lchihara, 1962). Prenatal fatty layers of ear plugs
from various kind of baleen whales are indicated Plate V; Figs. I, 2, 3, 4 & 5.
The longitudinal fatty layer is absent in the keratinized layer of the prenatal
ear plugs. In the bisected specimen of the ear plug from the adult baleen whale,
the longitudinal fatty layer is clearly present at the concavity in the epidermis corresponding to the undulate papillae in the derma of the glove-finger. In the postnatal ear plug, furthermore, the longitudinal fatty layer becomes more clear near
the proximal end with the advancement of the whale age. This is primarily because
the papillae of derma project outwards with the increasing age of the whale, although
there are individual variations in its degree. The absence of the longitudinal fatty
layer in the prenatal stage is based on not only the comparatively flat surface ofpapillae but also the unique process in which the keratinized fusiform cell is nearly

42

T. ICHIHARA

the same size as the fatty degenerated cell. This feature is shown in Plate IV, Fig.
2. The transformation of degenerated cell in the ear plug from the adult whale
was already described (see Fig. 9, Ichihara, 1959). The physical force during thedevelopment of ear plug effects intensively on the formation of the longitudinal
fatty layer. This will be examined in connexion with the renewal time of epidermal cells in the future study.
DISCUSSION

Laws & Purves (1956), examing ear plugs from young fin whale captured at
the norweigian coast, confirm their earlier conclusions about initial stages in theformation of the plug. They continue their statements that the first lamination
which is always a small, conical, unstratified body rich in calcium, is deemed
to be that part of the plug which is present in the ear before birth. It is vague
what they mean by the first lamination, but the unstratified body probably
indicates only the fatty layer described in the previous chapter of this paper. There
are, however, two layers in the ear before birth, composed of dark and brightlayers.
Chittleborough (1959) reports that only the secretary membranes were foundwhen the ear plugs from near-term humpback whale foetuses were dissected out,
and he considers that the first lamination of the ear plug is laid down soon after birth.
Although he disagrees with the presence of prenatal ear plug in the humpback whale,
I suppose that his finding depends on examing the surface of the glove-finger onwhich the collapse of prenatal keratinized and fatty layer has already progressed.
Vith the increasing time from kill to Bensing, the new ear plug rich in keratin undergoes decomposition and hence it is disconnected from the surface of the glove-finger,even in adult baleen whales. I confirmed the presence of the prenatal ear plug fora near-term humpback whale foetus taken off Ryukyu Islands, as reported in myprevious paper (lchihara, 1959). Besides in this paper I reconfirm the occurrenceof degenerated epidermal cells in glove-fingers from two Antarctic humpback whale
foetuses. Crisp (1963) discusses the effect of adult whale length and post-mortem
time on ease ofremoving ear plugs from fin whales and concludes that there is a significant difference in the success of extraction between the whales with post-mortem timesof less than 7 hours and those that had post-mortem times of more than 7 hours.
There is a positive relationship between the ease of removing ear plugs and the decomposition of their tissues, however, the technical ease is different from the accurate examination on the ear plug. Histological examinations need the freshmaterials.
The development of baleen plate should be noticed here. The periodic growthof the cortical layer sheathing the medullary layer in the baleen plate has been an
indicator of age of baleen whales since Ruud (1940) pointed its significance. The
cortical layer is formed by the cornification of cells from the lateral surfaces of theblade-pulp, while the medullary tissue is formed from the lower and inner margin(Eschricht & Reinhardt, 1866; Tullberg,1883). Citing Tullberg's paper, Mackin'1

PRENATAL EAR PLUG

43

tosh and Wheeler (1929) states that the first rudiments of the two blocks of baleen
plate are found after the blue whale foetus reaches a length of 2 metres or more
and then two plain strips of a soft whitish material appear, one on each side of theupper jaw. They state, furthermore, that at the 2.5 to 3 metres, minute transverse
ridges appear on these strips and later develop into whalebone plates. With regard
to fin whales, Mackintosh & Wheeler describe that the development of the baleenin the blue whale foetus applies equally to its develpoment in the fin whale foetus.
Ohsumi (1960) reports that the first rudiments of the baleen plates are found at thelength of 1.75 metres in the Antarctic fin whale foetus.
According to the foetal growth curve of baleen whales, indicated by Laws
(1959), the length of 2 metres in the blue whale and that of 1.75 metres in the fin
whale correspond to the 7th foetal months. The occurrence of rudiments of baleen
plates does not yet indicate the keratinization of cells from lateral surfaces of theblade-pulp.
The evidence that the keratinized cells in the epidermis of the glove-finger
appears at the 6 l /2 foetal month of the fin whale supports the finding that steps inthe prenatal development of keratinized cells depend on various tissues of the whale
body. The keratinization might be more precocious in the epidermal cells of the
glove-finger than those of the other tissues, however, it is necessary to reexamine thesignificance underlying in the comparison between the periodic mark on the baleen
plate and the laminae number in the ear plug, because these two age indicators forbaleen whale is formed primarily from the keratinization of cells.
In the prenatal stage of fin whale, the fatty degenerated cells appears from the
8th foetal months and develops until birth. The prenatal fatty layer in the otherbaleen whale enables us to discriminate it from the postnatal accumulation of the ear
plug. A sudden turn of the epidermal cells from the keratinized to the fatty degeneration can not occur without the internal stimulation of the body. In the foetal
stage, it is the most reasonable to accept the endocrinal stimulus through the blood
supply from the maternal body. Although enzymes act as catalysts in the regulationof metabolic processes, hormones in vivo exert their influence by acting indirectly
upon the enzymes which catalyze the particular reaction (Rothman, 1954).
The season when the fatty degeneration arises in the maternal ear plugs will be
examined in relation to the physiologicals rhythm in the future study, throughwhich the relation between the biological clock and the calendar period vlill beclarified.
The neonatal line of the ear plug is defined as the part where the formation of
the prenatal fatty layer ends.
SUMMARY

In the course of my continuous study on the accumulation of the ear plug, the mostreliable material to determine the age of baleen whales, I examined the prenatal
development of ear plug by using foetal specimens from 48 fin whales, 17 sei whalesand 2 humpback whales in the Antarctic. The foetal growth curve by Laws (1959)

44

T. ICHIHARA

is applied in this paper to obtain the accurate foetal month for the examined specimens. Through the naked eye observation during dissecting the external auditory
meatus from the blind section to the glove-finger and through the histological examination on the celloidin section of the glove-finger, my findings are summarized
as follows.
I. The shape of prenatal ear plug is determined by the difference in growth
between the external auditory meatus and the glove-finger in the foetal stage ofbaleen whale. With regard to shapes, there are individual variations in the same
species as well as the specific differences among baleen whales.
2. The prenatal ear plug is essentially maintained throughout the life span of
the baleen whale. It is observed clearly in the ear plugs from the adult blue, fin,
Bryde's and humpback whales but rarely in the ear plugs from adult sei and minke
whales. In the sei and minke whale, the connection between the ear plug-core andthe outer-covering is so weak that the ear plug is separated at the distal portionof the core in the course of removing ear plugs from the whale bodies. This implies
some difficulties in age determination by ear plug for these two species.
3. The keratinization in the epidermal cells of the glove-finger appears at the
6 1/2 foetal month of the fin whale, according to the histological examination. It
arises at the 8 I /2 foetal month of the sei whale. The observation by the naked eyefor foetuses endorses this finding. The keratinized layer in the prenatal stage,
showing great individual variation of thickness, remains as the first dark layer atthe distal end of the plug-core.
4. The fatty degeneration in the epidermal cells of the glove-finger occul's at
the 8th foetal months and continues to progress until birth in the fin whale. It also
appears at the late foetal month of the sei whale. In the adult ear plugs frombaleen whales, the fatty layer remains as the first bright layer following the above
keratinized layer.
5. The neonatal line of the ear plug is defined as the part where the firstbright layer ceases to grow.
6. The longitudinal bright layer is not yet present in the foetal ear plug.
7. The substances stimulating the foetal epidermis of the glove-finger to thefatty degeneration seem to be supplied through the maternal blood. The endocrine
factors may act an important part.
REFERENCES
Chittleborough, R. G. ( 1959). Determination of age in the humpback whale, Megaptera nodosa (Bonnaterre).
Aust. J. Mar. Freshw. Res., 10: 125-143.
Crisp, D. T. (1963). The effect of whale length and post-mortem time on ease of removing ear plugs fromfin whales. Norsk Hvalfangst-Tid., 5: 121-123.
Eschricht, D. F.,&]. Reinhardt (1866). On the greenland right whale. Recent memoirs on the Cetacea.
Roy. Soc. of London.
Ichihara, T. ( 1959). Formation mechanism of ear plug in baleen whales in relation to glove-finger. Sci. Rept.
lJ7hales Res. Inst., 14: 107-135.
- - (1963). Photometric method for counting laminae in ear pug of baleen whale. Sci. Rept. Whales
Res. Inst., 17: 37-48.

PRENATAL EAR PLUG

45

Laws, R. M. & P.E. Purves (1956) The ear plug of the Mysticeti as an indication of age with special reference to the North Atlantic fin whale (Balaenoptera physalus Linn.) Norsk Hvalfangst-Tid., 8: 413-35.
Laws, R. M. (1959). The foetal growth rates of whales with special reference to the fin whale, Balaenoptera
physalus Linn. Discovery Repts., 29: 281-308.
Mackintosh, N. A. &J. F. G. Wheeler (1929). Southern blue and fin whales. Discovery Repts., 1: 257-540.
Matthews, L. H. (1938). The sei whale, Balaenoptera borealis. Discovery Repts., 17: 183-290.
Montagna vV. (1962). The structure and function ef skin. Academk press, New York.: 1-454.
Nishiwaki, M., T. Ichihara & S. Ohsumi (1959). Age studies of fin whale based on ear plug. Sci. Rept.
Whales Res. Inst., 13: 155-169.
Ohsumi, S. (1960). Relative growth of the fin whale, Balaenoptera physalus (Linn.) Sci. Rept. Whales Res.
Inst., 15: 17-84.
Purves, P. E. (1955). The wax plug in the external auditory meatus of the Mysticeti. Discovery Repts., 27:
293-302.
Purves, P. E., & M. D. Mountford (1959). Ear plug laminations in relaion to the age composition of a
population of fin whales (Balaenoptera physalus). Bull. Brit. Mus., (Nat. His). Zool., 5: 125-61.
Rothman, S. (1954). Physiology and biochemistry of the skin. The University of Chicago press. Chicago; 1741.
Ruun, J. T. (1940). The surface structure of the baleen plates as a possible clue to age in whales. Hvalradets
Skrifter., 23: 1-23.
TuLLBERG, T. (1883). Bau und Entwicklung der Barten bei Balaenoptera sibbaldii, Nova Acta Soc. Sci. Upsal.,ser. III, 11 : 1-36.

46

T. ICHIHARAAPPENDIX.

THE BIOLOGICAL DATA FOR EXAMINED FOETUSESFIN WHALE

Dateobtained

No.

Foetallengthin cm

Foetalmonth

Sex infoetusElapsedtime
No. of ovulaaftertion in mothkilling er's ovaries Position captured

1
2
3
4
5

9, Jan.
4, Jan.
6, Jan.
26, Jan.
7, Jan.

1960
1960
1960
1960
1960

6.6
19.0
47.5
52
71

3/4-1
1-3/4
2 3/4-33
3 3/4-4

F
F
F
M
M

9-20
12-40
11-10
7-10
5-15

11
25
25
7
13

56°-I9 1S, 94°-561 E
57°-0718, 100°-231E
56°-3415, 95°-09 1E
60°-55 1S, 153°-01 1 E
56°-26'5, 92°-57 1 E

6
7
8
9
10

29, Dec.
29, Jan.
28, Dec.
8, Jan.
30, Dec.

1959
1960
1959
1960
1959

79
117
122
122
127

4 1/4
5 1/4
5 3/4-6
5 3/4-6
6

F
F
M
M
F

7-40
8-25
6-20
11-35
2-55

2
4
14
5
9

58°-3618, 92°-27 1 E
59°-31'5, 150°-!41 E
59°-12'S, 94°-241 E
57°-0018, 93°-361E
57°-5415, 93°-45'£

11
12
13
14
15

2, Feb.
23, Jan.
24, Jan.
30, Jan.
9, Mar.

1960
1960
1960
1960
1960

130
140
145
145
145

6-6 1/4
6 1/4-6 2/4
6 2/4
6 2/4
6 2/4

M
F
F
F
M

7-50
6-50
8-15
12-15
5-00

I
22
13
11
7

59°-321S, 150°-47 1E
62°-2l'S, 146°-l21 E
61°-4315, l50°-l7 1E
59°- I 41S, I 48°-441E
60°-541S, 150°-571E

16
17
18
19
20

10, Jan.
12, Feb.
31, Jan.
2, Feb.
11, Jan.

1960
1960
1960
1960
1960

165
173
175
175
183

6 3/4-77
7-7 1/4
7-7 1/4
7 1/4

M
M
M
F
M

11-40
11-00
6-15
1-35
9-35

7
I
2
5
3

56°-l61S, 92°-l91 E
68°-241S, 168°-l2'W60°-4818, 148°-521 E
59°-321S, 150°-47 1E
57°-l91S, 9! 0 -291E

21
22
23
24
25

21, Jan.
22, Jan.
19, Jan.
28, Dec.
30, Jan.

1960
1960
1960
1959
1960

183
183
191
203
206

7 1/4
7 1/4
7 1/4-7 2/4
7 2/4
7 2/4-7 3/4

F
F
F
M
M

7-45
12-45
10-10
5-25
10-50

3
2
21
19
2

63°-l61S, 140°-231E
62°-l21S, 144°-061E
64°-0618, 126°-321 E
59°-121S, 94°-241E
59°-l41S, 148°-44 1E

26
27
28
29
30

29, Dec.
21, Jan.
30, Jan.
. 4, Jan.
4, Feb.

1959
1960
1960
1960
1960

219
242
252
259
264

7 3/4
8-8 1/4
8 1/4
8 1/4
8 1/4-8 2/4

M
F
F
F
F

5-50
7-05
10-30
4-15
4-55

14
19
2
13
9

58°-37'S, 92°-26'E
63°-I61S, 140°-231 E
59°-I41S, 148°-44'£57°-07 1S, 100°-231£
59°-48'5, 150°-241 £

31
32
33
34
35

25, Jan.
30, Dec.
23, Jan.
4, Feb.
12, Jan.

1960
1959
1960
1960
1960

269
280
280
292
300

8 2/4
8 2/4-8 3/4
8 2/4-8 3/4
8 3/4
8 3/4

M
M
F
M
F

10-05
3-25
5-55
6-05
7-30

2
12
12
13
27

60°-38'5, 153°-561£
57°-541S, 93°-451E
62°-2! 1S, 146°-l21 E
59°-48'S, 150°-241 E
57°-00 1S, 92°-57 1E

36
37
38
39
40

18, Jan.
28, Dec.
28, Jan.
29, Jan.
29, Jan.

1960
1959
1960
1960
1960

300
323
338
348
353

8 3/4
9
9 1/4
9 1/4
9 1/4-9 2/4

M
M
F
M
F

11-55
3-15
5-55
10-00
12-50

16
19
15
17
20

64°-191S, 122°-101£
59°-!21S, 94°-241 £60°-35'5, 150°-361E
59°-3l'S, 150°-l4'E
59°-31 1S, 150°-!4'E

41
42
43
44
45

1, Mar.
25, Jan.
4, Feb.
9, Mar.
9, Mar.

1960
1960
1960
1960
1960

353
374
384
424
437

9 1/4-9 2/4
9 2/4
9 2/4-9 3/4
10
10

M
F
F
M
F

10-20
5-00
5-55
13-35
8-55

9
10
13
4
7

65°-05 1S, 17! 0 -48'E
60°-38'S, 153°-561E
59°-481S, 150°-24'E
60°-541S, 150°-571 E
60°-54'S, 150°-571£

46
47
48

22, Mar. 1960
11, Mar. 1960
22, Mar. 1960

440
531
556

10-10 1/4
10 2/4-10 3/4
10 3/4-11

F
M
F

1-25
2-20
2-05

4
29
17

59°-3! 1S, 150°-40'E
59°-3 l'S, I 50°-35'E
59°-3 l'S, 150°-40'E

M: MaleF: FemalePRENATAL EAR PLUG

47

SEI WHALE

No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

FoetallengthDateobtained
25, Jan.
12, Feb.
7, Feb.
29, Jan.
5, Feb.
7, Feb.
13, Feb.
25, Jan.
27, Jan.
2, Feb.
8, Feb.
12, Feb.
27, Feb.
8, Feb.
22, Feb.
10, Feb.
30, Jan.

in cm

Foetalmonth

Sex infoetus

84
153
175
183
193
198
198
242
247
249
264
300
305
315
315
323
325

5-5 1/4
7 1/4-7 2/4
8 1/4-8 2/4
8 2/4
8 2/4
8 3/4
8 3/4
9 1/4-9 2/4
9 2/4
9 2/4
9 3/4
10 1/4-10 2/4
10 1/4-10 2/4
10 2/4
10 2/4
10 2/4-10 3/4
10 2/4-10 3/4

M
F
M
M
M
M
M
F
F
M
F
M
M
F
M
M
F

1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960
1960

Elapsedtime
No. of ovulaaftertion in mothkilling er's ovaries
Position captured
9-40
5-25
3-30
8-30
6-05
3-40
5-50
9-00
7-15
9-05
2-45
8-05
0-45
3-50
3-50
4-45
9-35

8
4
4
10
13
8
11
6
13
6
4
6
5
9
10
13
5

60°-38'S, 153°-56'E
68°-24'S, 168°-12'W
65°-lO'S, 165°-00'E
59°-3l'S, 150°-14'E
60°-29'S, 153°-46'E
65°-lO'S, 165°-00'E
69°-0l'S, 166°-ll'W
60°-38'S, 153°-56'E
60°-19'S, 148°-42'E
59°-321s, 150°-47'E
66°-53'S, 169°-22'E
68°-24'S, 168°-12'W
66°-43'S, 178°-54'W
66°-53'S, 169°-22'E
65°-32'S, 163°-041W
69°-31 1s, 179°-00'W
59°-141s, 148°-44'E

HUMPBACK WHALE

No.

2

Date
obtained

Foetallengthin cm

Foetalmonth

Sex infoetusElapsedtime No. of ovulaaftertion in mothkilling er's ovaries
Position captured

21, Jan.

1960

109

6 1/2

M

3-45

8

63°-16'S, 140°-23'E

21, Jan.

1960

125

6 3/4

M

7-05

8

. 63°-16'S, 140°-23'E

M: MaleF: Female

EXPLANATION OF PLATESPLATE I
Fig. 1. The cross section of the glove-finger from the 3 3/4-4th foetal months of the
fin whale ( x 400). The compact basal layer, covered by periderm cells, some ofwhich are exfoliating. The basal layer is composed of low cuboidal cells.
P:
Periderm
BL : Basal layer
CT : Connective tissue
Fig. 2. The cross section of the glove-finger from the 8 1/2-8 3/4 foetal months of the
fin whale ( x 100). The thick epidermis is much proliferated. The keratinizedlayer is disconnected from this specimen.
BL : Basal layer
CT : Connective tissue

48

T. ICHIHARA
PLATE II
Fig. 1. The cross section of the glove-finger from the 9 1/4-9 1/2 foetal months of
the fin whale ( x 100). The keratinized layer darkly stained by eosin, is clearlyfound at the most outer side of the epidermis. The most part of keratinized layer
is disconnected from this specimen.
KDS : Keratinization darkly stained by eosin
BL :Basal layer
The enlarged photomicrograph for the same specimen ( x 300).

The lipoid

granules are found in the cystoplasm of the upper prickle-cell layer.

There areFig. 2.

many transofrmed nuclei.
FC : Fatty degenerated cell
PLATE III
Fig. I. The cross section of the glove-finger from the lOth foetal months of the fin
whale ( X 100). The fatty degenerated layer is clearly observed. The most part ofkeratinized layer is disconnected from this specimen.
KDS : Keratinization darkly stained by eosin
BL :Basal layer
Fig. 2. The enlarged photomicrograph for the same specimen ( X 400). Swelledcells, undergoing the fatty degeneration, are found at the upper prickle-cell layer.
At the most outside of the fatty layer depressed nuclei remain yet. The keratinizedlayer darkly stained by eosin is found at the most distal end.
FC : Fatty degenerated cell.
FL: Fatty layer.
PLATE IV
Fig. 1. The epidermis from the 10 3/4-11 th foetal months of the fin whale ( X 100).
The lower parts of keratinized layer is on the thick fatty layer.
KL: Keratinized layer
FC : Fatty degenerated cellFig. 2. The enlarged photomicrograph for the same specimen indicates the fatty
degenerated cells at the prickle-cell layer ( X 400).
the same size as the fatty degenerated cell.
KC : Keratinized cell
FC : Fatty degenerated cell.

The keratinized cell is about

PLATE VThe prenatal layers remaining at the most distal ends of plug-cores from adultbaleen whales. Prenatal layers are composed to two layers: keratinized (KL) andfatty degenerated (FL) layers.
Fig. 1. Fin whale ear plug
2. Blue whale ear plug
3. Humpback whale ear plug
4. Sei whale ear plug
5. Bryde's whale ear plug
PKL : Keratinized layer of the prenatal stage
PFL : Fatty layer of the prenatal stage

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