OBSERVATIONS ON THE OVARY OF
THE SOUTHERN MINKE WHALECHRISTINA LOCKYER*
ABSTRACT

Ovaries of minke whales appear similar in shape, form and structure
to those of related balaenopterid whales. However, the size and weight ofthe ovaries and their internal structures are smaller. In particular, themean sizes of the active corpus luteum (CL) and regressed corpora albicantia (CA) are smaller. A high number of corpora appear to be present in theovaries relative to age, consistent with a more frequent ovulation rate and/or shorter reproductive interval than observ~d in other balaenopterids.
The types of corpora observed in the minke whale are similar to those reported for other whales, although the CA in minke are characteristicallyvery yellow in colour. The CA appear to persist in the ovary despite considerable shrinkage and degeneration, thus forming a complete record of
the individual's ovulation history. The cyclical changes in the ovary in relation to reproductive condition appear similar to those observed in otherbalaenopterids.
INTRODUCTION AND BACKGROUND

The minke whale, Balaenoptera acutorostrata, the smallest of the balaenopterid
cetaceans, attaining a maximum size in the southern hemisphere of 10. 7 m(Lockyer, 1984) and weight of about 13.87 t (Lockyer, 1976; Ohsumi, 1979),
is found in all latitudes of the world's' oceans. Although the life history,
longevity, pattern of feeding, growth and reproduction have many similarities
with other balaenopterids, the duration of the reproductive cycle appears todiffer, and in minke whales is closer to one year rather than two years as in
the majority of this genus (Best, 1982; Christensen, 1974; lvashin and
Mikhalev, 1978; Jonsgard, 1951; Larsen and Kapel, 1983; Lockyer, 1981,
1984; Masaki, 1979; Mitchell and Kozicki, 1975; Williamson, 1975).
The most reliable estimate of reproductive cycle length is 14 months(IWC, 1979), comprising 10 months' gestation and 4 months' suckling of asingle calf. The female appears to commence a subsequent pregnancy either
during lactation or at weaning of the calf. The evidence for this is derived
from ovulation rates calculated by plotting ovarian corpora numbers versus
age, from foetal growth records and timing of parturition, and from propor-* Sea Mammal Research Unit (NERC), c/o British Antarctic Survey, Madingley Road, High Cross, Cambridge CBJ
OET, England.
Sci. Rep. Whales Res. Inst.,
No. 38, 1987, 75-89

76

LOCKYER

tions of pregnant, lactating and anoestrous females in the mature portion of
the commercial catches.
Studies of minke whale populations have been made in both hemispheres, and include polar waters of the Antarctic, off Norway and Greenland,
and more temperate seas off South Africa, Canada, Japan and Korea, as wellas equatorial waters off Brazil. The biology of the different populations has
generally been comparable.
The subject of the present study is the Antarctic minke whale. The
mature female usually conceives in August-September, and gives birth in
May-June of the following year to a 2.8 m neonate (Ivashin and Mikhalev,
1978; Lockyer, 1984). The mean age at first ovulation and possibly conception, ranges between 6-14 years (Masaki, 1979; Kato, 1983, 1985) at an
average body length of 8.0 m and weight of 6.05 t. In this study, a detailed
account is presented of the macroscopic structure of the ovaries of the minke
whale, and analyses of cyclical activity in relation to reproductive age andstatus.

MATERIAL AND METHODS

The study material comprised 67 pairs of ovaries, collected during theJapanese Antarctic minke whaling expedition December 1980-March 1981.
The ovaries were removed intact from freshly killed carcases on board factory
ships, were maintained frozen between - l5°C and -20°C, and shipped backto Japan where, after initial palpation and examination at the Far Seas
Fisheries Research Laboratory, Shimizu, they were fixed and preserved wholein neutral buffered 10% formalin (4% formaldehyde) and freighted toCambridge. On arrival, the ovaries were in excellent anatomical and histological condition. Data on sexual status and condition, age, date of capture, bodylength, geographical position of capture and foetal length (if present), were
provided. The classification of the reproductive status, e.g. pregnant, anoestrous, lactating, was determined on site when the whale was first dismemberedafter capture and the ovaries collected. This was done by careful macroscopic
investigation of the uterus and mammary glands. Ages of individuals were
determined by Dr H. Kato (Whales Research Institute, Tokyo).
After soaking/rinsing in running water for 24-hours, the individual
ovaries were trimmed of adhering fallopean tube and connective tissue,
weighed and examined for gross general appearance and presence of corpora
lutea. These latter, if found, were detached, weighed, and cut vertically fromthe ovulatory scar along two planes at right angles to each other and to a third
transverse plane of cut. The maximum diameters through these three planes
were measured to give an average diameter of the corpus luteum (CL).
The ovaries were then sectioned at intervals of 2-3 mm, using a commercial meat slicing machine. During this procedure, slices were accumulated
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MINKE WHALE'S OVARY

77

serially and examined for the presence of corpora albicantia (CA), follicles
and various other ovarian structures, all of which were recorded and measured
for mean maximum diameter in at least two planes. Unusual items, such as
corpora atretica, were excised and put aside in 10% formalin for subsequent
histological examination for confirmation of their identity.
The classification of ovarian bodies was made by reference to work ofLaws(l961) and the International Whaling Commission report (IWC, 1984).
The categorization of the CA into young, medium and old in terms of state ofregression and resorption has been based on the terminology of Laws( 1961).

RESULTS AND DISCUSSION

The ovarian structure is typically mammalian (Mossman and Duke, 1973),
and generally conforms to the findings of Harrison (.1969), Laws ( 1961) and
Slijper (1966) for other cetaceans. In the following analyses, the ovaries were
sorted by reproductive category of the female, as defined below:
Immature-where ovaries contain follicles but no evidence of ovulation;
Mature-where ovaries, of which at least one half of the pair, containevidence of ovulation; there are sub-categories of maturity status:a. Pregnant-where ovaries contain an active corpus luteum, and there isevidence of foetal development in the uterus;b. Ovulating-where ovaries contain a large active corpus luteum, butthere is no evidence of a foetus in the uterus; note however, that fertilisation may have occurred but not implantation;c. Regressing CL-where ovaries contain a no longer active corpus
luteum which is undergoing shrinkage and histological changes commensurate with becoming a corpus albicans (Marsh, 1985; Marsh andKasuya, 1984);
d. Anoestrous -where ovaries contain no corpora lutea, active or regressmg;
e. Lactating-where milk is present in the mammary glands, but theovaries may contain a regressing CL, an active CL or no CL; ovulationand pregnancy may occur during lactation.
Gross appearance of the ovaries
1) Size
The elongate, slightly flattened ovaries are lobulate in mature andmaturing females and smooth in juveniles. The overall external colour is a
mixture of pink, white and grey. The hilus is long, similar to that in the fin
whale, B. physalus (Laws, 1961). Corpora (both CL and CA) generally protrude from the surface, and are connected at the base by a thick "neck" to themain body of the ovary. The ovary surface is frequently criss-crossed by "tags"
and adhesions such as observed for sei whales, B. borealis (Gambell, 1968 andSci. Rep. Whales Res. Inst.,
No. 38, 1987

78

LOCKYERTABLE I. OVARY WEIGHTS IN MINKE WHALES
Reproductive classand category
Immature
MatureAnoestrous < 1OCA
ll-24CA>25CA
All groups
Pregnant All groups
Ovulating (CL+ l 9CA)*)
Regressing CL (2-1 lCA)**)No individual
ovaries

Mean ovary weight(g)±s.d.
Ovary without CL
Ovary with CL

2250.4±37.92

10
12
8
30
36

77.75±31.72
167.50±45.40
218.12±97.24
150.11±81.13
161.14±68.62
200
78.17±5.012
6339.17± 113.16
480
131.67± 16.07•) corpora count of ovary pair,**) range of corpora counts in the 3 ovary pairs.

personal observation); 18% of juveniles and 12.5% of mature females showing this feature. Sometimes mushroom-like appendages dangle from themain body' of the ovary, but have no unusual internal structure. These havealso been observed in fin whales (Laws, 1961 ).
The paired ovaries are usually of similar size and weight, except where a
CL is present, when the CL-bearing ovary greatly exceeds the weight of theother.
2) Weight
The average weight of ovaries for different reproductive classes areshown in Table 1. The minke ovaries, by reproductive class, are almost one
tenth the weight of those reported for the much larger fin (Laws, 1961) andsei whales (Gambell, 1968). As expected, the immature ovaries are lightest. In
the mature ovary, weight increases significantly with the accumulation of CA,
although the overall mean weight of the anoestrous ovary is similar to that of
the non-CL-bearing ovary of the pregnant female. The combined effects ofcorpora number and stage of pregnancy have not been investigated, as sample
size is inadequate. The CL-bearing ovary in both the fully pregnant and postlactational phase (with regressing CL), is significantly heavier than its partner.
This, as will be shown later, is almost certainly due to the additional weight of
the CL. The single ovulating female in the sample, also demonstrates a weight
assymetry with greater weight in the recently ovulated ovary.

Internal structure of the ovary
Internally, the encapsulating tunica albuginea in mature ovaries is
between 0.83-2.99 mm in thickness with a modal range 1.25-1.50 mm. This iscomparable to the 0.95-1.60 mm observed in fin whales (Laws, 1961). The
Sci. Rep. Whales Res. Inst.,
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79

MINKE WHALE'S OVARYTABLE 2. MEAN DIAMETER OF THE LARGEST FOLLICLES IN MINKE WHALES
Reproductive classand category
Immature(age 2-7 years;
length 6.2-8.6 m)
MatureAnoestrous
Pregnant(foetus <30 cm)(foetus 30-130 cm)(foetus> 130 cm)
OvulatingRegressing CL (2-1 lCA)*l

No females

Mean diameter of the largest
follicles (mm) ±s.d.

116.41±3.35179.29±4.908262

9.87±5.92
13.74±3.81
13.00±0.07
14.00
3.50±2.18

I

3•) range of corpora counts in the 3 females.

body of the ovary, particularly the medulla, is well supplied with blood vessels, and the cortex contains varying numbers of follicles of differing sizes
with developing oocytes, the stage dependent on reproductive condition of
the female, and products of ovulation, the CL and CA. In addition, various
other bodies are found near the cortical surface, including different typesof corpora atretica, cysts and cystic follicles. These ovarian bodies aredescribed below.
1) Follicles
The dimensions and abundance of gel-filled Graafian follicles, foundthroughout the cortex of the ovary, vary in size with stage of maturity, andwith stage of the reproductive cycle (Table 2). The immature ovaries ofjuveniles and animals approaching puberty have smaller follicles than thosein most mature ovaries. In very young females, the follicles are not macroscopically visible.
Ovulating and mid- and late-term pregnant females have the largest
follicles, whilst follicles in ovaries of anoestrous and early-term pregnant
females are smaller. The smallest follicles, indicating little follicular activity,
are seen in the ovaries of females with a regressing CL.
Follicular activity in terms of size, throughout the reproductive cycle inminke, is broadly similar to that of fin whales (Laws, 1961).
No mature ovaries were observed to lack follicles as has been reported by
Marsh and Kasuya (1984) for older "post-reproductive" or senescent pilot
whales (Globicephala macrorhynchus).
2) Corpus luteum
The active CL is similar in appearance to those described for fin (Laws,
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80

LOCKYERTABLE 3. SIZE OF THE CORPUS LUTEUM IN MINKE WHALES
Reproductive classand category

Mean size of CL±s.d. (n=sample size)
Diameter (mm)Weight (g)Pregnant(foetus <30 cm)(foetus 30-130 cm)(foetus >130 cm)
OvulatingRegressing CL

62.68±8.78
66.40±6.79
70.58± 12.94
75.70
41.37±3.60

(8)
(26)
(2)
(1)
(3)

137.25±45.27 (8)156.73±48.39 (26)202.50±137.89 (2)180.00(1)
48.00±11.31 (3)1961 ), sei (Gambell, 1968) and blue whales, B. musculus (Mackintosh andWheeler, 1929); its form and structure is as described in the IWC report(IWC, 1984).
The CL of the ovulating female (determined as such from the absence ofa foetus) was similar in appearance to those of the pregnant females, confirming the view (IWC, 1984; Marsh, 1985) that the CL of ovulation andpregnancy are not readily distinguishable, if at all. Of course, during the preimplantation phase when the blastocyst is unattached, the reproductive statusof the female is impossible to differentiate from that of the condition with arecent unfertilised ovum.
The stigma, site of the follicular rupture at ovulation, was usually clearlyvisible at the apex of the protruding CL, and was often surrounded by aslightly raised rim or corona. The colour of the CL was pinkish-grey to yellow.
One unusual form of CL was found where a surrounding fold of tunica
albuginea from the ovary had formed around the body, enfolding it like a cup
around an acorn. Central vesicles within the CL were observed in 25% of
early-pregnant (n=8; foetus length<30 cm) and 33.3% of mid-pregnant
females (n=26; foetus length 30-130 cm), but none were observed in latepregnant females (n=2; foetus> 130 cm). There was no significant association
with stage of pregnancy. Larsen (1984) reported an incidence of 13% ofvesicular CL in pregnant minke whales, but no breakdown by stage of pregnancy was presented.
Variation in size of the CL throughout the reproductive cycle is shown in
Table 3. Volume and weight appear to increase with advance of pregnancy.
However, the differences in size are not significant with these small samples.
The regressing CL, determined as such from shrinkage of the secretory
lobules and commencement of fibrous replacement of the luteal tissue, appears
very small in relation to the size at ovulation, and approaches the mean
largest size of the youngest CA.
The additional weight of the CL explains the assymetry in the weight ofovaries from pregnant females (Table 1).
The size range of the minke CL (Table 3) is similar to that observed byBest ( 1982) for minke whales off South Africa, and by Larsen and Kapel
Sci. Rep. Whales Res. Inst.,
No. 38, 1987

81

MINKE WHALE'S OVARYykey :o pregnant• anoestrous
t:..

40

regressing CLDo ovulating

D

•
D
D

D

D

30
D
D

•

...z
::>

D

0
u

O

<(

...0"'

D

"'0
u

D
D

D

•D
OD

D

•

••

0
...-~~~~~---,-~----,~~~~~~~~---,-~----,~~x

0

10

20

AGE

30

40

50

IN YEARS

Fig. I. Observed relation of ovarian corpora number with age from ear plug growthlayers in minke whales.

(1983) for the minke whale off Greenland. Larsen (1984) gave a mean size of
CL of pregnancy as 56.6±5.8(s.d.)mm for Greenland minke whales, rather
smaller than the values for the southern hemisphere. He considered that thesmaller size of the CL of tpe northern hemisphere animals might be due totheir smaller body size. The size of the CL of ovulation observed here seems
large by comparison with the data summarised by Lockyer (1984), approaching nearly twice the diameter given by Larsen (1984). However, no significance can be ascribed to this solitary observation. The mean sizes of the minke
CL are smaller than those of all other baleen whales quoted by Lockyer ( 1984)
for pregnant females, including:blue, 136.5 mm (Mackintosh and Wheeler, 1929),
fin, 114.4 mm (Laws, 1961),
sei, 76-84mm (Gambell, 1968),
humpback, Megaptera novaeangliae, 106-123 mm (Matthews, 1937;
Chittleborough, 1954),
Bryde's, B. edeni, 72-78 mm (Best, 1977), and gray, Eschrichtius robustus,
82-87 mm (Rice and Wolman, 1971).
In all these baleen species, the mean size of the CL of ovulation is smaller
than that of pregnancy.
No examples of accessory CL, as described by Laws (1961) and IWC
Sci. Rep. Whales Res. Inst.,
No. 38, 1987

82

LOCKYER(1984), were found. However, there is a possibility of their existence, particularly with reference to c. atretica described later, whose function and fate areunclear.
3) Corpora albicantia
All CA were classified into young, medium and old categories (Laws,
1961), and mean diameters measured. The first impression gained in examining the ovaries, was the high numbers of CA accumulated even in young
females, so that it was not unusual to observe many females of age 15 years
and over with as many corpora as years (Fig. 1). This suggests two points: 1)
females probably ovulate at least annually, and may be poly-ovulating; 2)complete resorption even of old CA is unlikely, so that CA are permanentfeatures of the ovary, even when greatly regressed in size.
The young CA, are in fact most usually highly pigmented and yellow-tan
in colour, whereas fin or sei ovaries are usually pale greyish-brown (personalobservation). In the minke whales, the medium and old CA generally becomebrown and greyish, but some CA are still well pigmented. The difference between a highly pigmented CA and a corpus aberrans (Laws, 1961) equivalent
to a corpus atreticum type a (IWC, 1984) (see Plate la) is not immediately
obvious, since the latter is often regularly shaped and bright yellow or orangeyellow in hue.
Fig. 2 shows the course of regression of the CA with time in different
reproductive classes.
4) Young CA
The mean maximum diameter range of the young CA is 28.14-16.50mm,with pregnant and ovulating females tending to bear slightly larger and, inthe former, more numerous young corpora than females in anoestrus andwith regressing CL, despite similar total counts of all ages of corpora. Thismight suggest that hormones of pregnancy may temporarily allay degenerative processes and shrinkage of the CA. Such explanations have been proposed for fin whales where similar observations were made (Laws, 1961 ).
Marsh and Kasuya (1984) found slower rate of shrinkage of CA in pregnant
pilot whales. Larsen (1984) gave a mean diameter of young CA of
21.4±3.5(s.d.)mm for Greenland minke whales, within the range presented
here.
Initially the CL shrinks rapidly to 40% of its original diameter (Larsen,
1984, gave a figure of 37.8%), and then continues to about 25% of original
CL diameter (Fig. 2a).
5) Medium CA
The range of mean diameters of the medium CA is 19.17-10.00 mm,
the smallest being about 15% of original CL diameter, and 40% of initial diameter of the youngest CA. The shrinkage of the CA in this medium phase isSci. Rep. Whales Res. Inst.,
No. 38, 1987

83

MINKE WHALE'S OVARY
b MEDIUM

a YOUNG~::~ \_. _, 1~

C OLD

~-~~-

Jj%:'"":~~ j~!~,-~~~
i~\1~
j"""fH+Ht+HIHl!i!o 'IHI 'I+

Pcegnant-partum

I

J

16

!

CORPUS,6;\5

NUMBERAND CLASSIFICATION

2bI', .bl\i''' 2b

"~'

ab

.~

Fig. 2. Regression of accumulated corpora albicantia, measured by mean diameter,in decreasing order of c.albicans size, by different reproductive classes:a. Young c.albicantia;
b. Medium c.albicantia;
c. Old c.albicantia.

slower (Fig. 2b), and there is considerable size overlap with the old CA category below (Fig. 2c). Larsen (1984) observed a shrinkage in diameter of
medium CA in Greenland minke to 15.8±2.4(s.d.)mm, similar to results here,
and 27.9% of original CL diameter.
6) Old CA
The range of mean diameters of the old CA is 18.5-7.5 mm. The shrinkage here is very slow (Fig. 2c), and not greatly different from that for medium
CA. The similar size range with the medium CA, suggests not so much continued shrinkage, but a continued histological change in the structure.
It seems unlikely that CA which have regressed to this third stage will regressor alter further; they become a permanent feature in the ovary. Examples of
old CA are shown in Plates la and ld.
The general pattern of corpus regression in minke whales is similar tothat in fin whales (Laws, 1961), but the sizes of minke CA in the different
ovarian age categories are smaller. The ultimate regressed modal diameter(about 10 mm) of the old CA in minke being approximately half that for thefin whale (Laws, 1961). However, this body is still quite readily visible macroscopically. Larsen (1984) found a mean diameter of old CA of
10.0±2.4(s.d.)mm in Greenland minke, and shrinkage to about 17.7% of original CL diameter. The equivalent shrinkage here is 15.2% of CL size, close to
Sci. Rep. Whales Res. Inst.,
No. 38, 1987

84

LOCKYERthe estimate of Best (1982) of 15.7%.
7) Corpora atretica, cystic follicles and cysts
Corpora atretica-type a: The corpus atreticum type a (IWC, 1984) identified here is equivalent to the corpus aberrans described by Laws (1961 ).
These bodies exhibit follicular origins, there being evidence of apparent
rupture or ovulation through presence of a stigma on the germinal
epithelium overlying the body (Plate lb). However, they fail to luteinise fully
and undergo instead a fatty degeneration over time (Plates le and ld). These
corpora often attain a moderate size, e.g. 23mm in diameter, and are very
bright yellow in colour (Plate le). When recently formed they appear as
lobules protruding from the mass of the ovary, and frequently a CA in form.
The presence of older regressed or atrophied ones is often ascertained only
after cutting into the ovary. A few maintain central vesicles (Plate lf).
No c.atretica a were found in the ovaries of the eleven juveniles, none in
the ovaries of the ovulating female, or of the three females with a regressing
CL. The anoestrous females (n=l4) showed an incidence of 78.6%. Half of
the anoestrous females had only a single c.atreticum a, while 7% had two,
14.3% had 3, and 14.3% had four. The mean diameter of the c.atreticum a
was 10.82±4.50(s.d.)mm. Among pregnant females, the incidence of c.atretica
a was 62.5% for those with a foetus <30 cm (n=8), 37.5% having a single
body and 25.0% having two. The mean diameter was 10.08±3.78(s.d.)mm.
The incidence in the group of foetal size 30-130 cm (n=26), was lower, at
26.9%, with 15.4% having a single body, 7.7% having two and 3.8% having
three. The mean diameter was 10.50±5.4(s.d.)mm. The two females in latepregnancy had no c.atretica a. Sample size is too small in several categories
for a valid x2-test, but restricting the test to incidences in anoestrous andpregnant females, x2= 10.6751 with d.f.=2, and is significant at p<0.01. It istherefore suggested that these bodies are directly associated with certain
phases of the reproductive cycle, and by implication, certain hormone levels.
Laws (1961) observed a decrease in incidence of c.aberrantia (c.atretica a) withadvance of pregnancy in fin whales. In odontocetes, Best ( 1967) reported thehighest incidence of c.atretica a in recently ovulated females, 45.5%, withlower levels of 4.6% in pregnant, 7.7% in lactating and 13.6% in anoestrous
sperm whales, Physeter macrocephalus. Marsh and Kasuya (1984) found an
apparent association of incidence of c.atreticum a with states of oestrus, ovulation and early pregnancy in pilot whales. The bodies appeared to degenerate
post-partum and in anoestrus.
The fact that percentage occurrence varies so much between reproductive classes in minke whales also suggests that most c.atretica a degeneratecompletely, perhaps within a year, the usual reproductive interval. Laws
( 1961) held a similar view on the fate of c.aberrantia (c.atretica a) in the fin
whale.
Of females with ovaries having a vesicular CL (n= 11), 54.5% had aSci. Rep. Whales Res. Inst.,
No. 38, 1987

MINKE WHALE'S OVARY

85

c.atreticum a, compared with 24.0% of those females with ovaries having a
non-vesicular CL (n=25). The difference, however, was insignificant with
p>0.05 using a x2test.
Corpora atretica-type b: This category corresponds with the definition ofc.atreticum used by Laws (1961) and with the c.atreticum type b (IWC, 1984).
These bodies are usually smaller than the c.atreticum a, and, unlike the frequently regular internal shape of the latter, are usually stellate or amoeboid inshape and deep yellow-orange or tan in colour. They never protrude fromthe ovary surface, and none examined exhibited a stigma. Usually, thesec.atretica b lay deeper in the cortex than the c.atretica a.
No c.atretica b were observed in females with immature ovaries (n= 11),and none were observed in the ovulating ovary or in the three pairs of ovaries
with regressing CL. The incidence in anoestrous females (n= 14) was 35.7%,
with 14.3% having one, 7.1 % having two, and 14.3% having seven c.atretica
b. The maximum diameter of the c.atreticum b was 8.3mm, but many weremuch smaller. Among pregnant females, the early pregnant (n=8) group had
an incidence of 12.5% with two c.atretica b, and very small diameter, <5 mm,
while mid-pregnant females (n=26) had an incidence of 34.6%, with 19.2%
having one and 15.4% having two c.atretica b. The maximum diameter observed was about 7 mm. Marsh and Kasuya (1984) observed no c.atretica bin
immature pilot whales, but found 75% of all mature ovaries bore thesebodies. Best ( 1967) found an average incidence of 50% in all reproductiveclasses of sperm whales.
Using similar criteria as above for the c.atreticum a data, a x.2-test
showed no significant differences between reproductive states, excluding
juveniles. A similar finding was obtained in fin whales (Laws, 1961), and alsoin pilot whales (Marsh and Kasuya, 1984) and sperm whales (Best, 1967).
The persistence of these bodies is uncertain, 'particularly because of theirsmall irregular form which may mean that some are inevitably overlooked ina macroscopic examination of the ovaries. Marsh and Kasuya (1984) suggestthat these bodies do not persist. Total resorption is a possibility. Also, theoverall incidence of c.atretica b are lower than for c.atretica a.
8) Cysts
Follicular, gel-filled cysts: In the present sample of ovaries, this type of cyst
appears to occur exclusively in ovaries of pregnant females, with an incidenceof 12.5% and a mean diameter of 31.7 mm in females in early pregnancy(n=8), and 3.8% and a mean diameter of 40 mm in females in mid pregnancy(n=26). No other examples of cystic follicles were observed. These cysts were
often multichambered, resembling a thin-walled honeycomb filled withmucous gel. The ultimate fate of such cysts is not known, but presumably
they regress, because they are not observed in other reproductive groups.
Should some luteinisation or hyalisation occur, they may regress to a form ofyellow body such as c.atreticum b, but this is speculative.
Sci. Rep. Whales Res. Inst.,
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86

LOCKYER

Cornified solid cysts: Only one example of a solid cyst was observed, andthis was present in the cortex of a female in early pregnancy, in the form of asmall whitish and homogeneous sphere, with a fibrous collagen-type of
texture. The origin of this body was unknown.
Differential activity in the ovary pairs
The side of the whale from which the ovary was collected was not generally known or identifiable. The possibility of bi-lateral activity could thus not
be evaluated as to preferential left or right activity. However, a simple analysis
was performed to identify whether or not one ovary, regardless of side, might
be more active than the other.
The pairs of ovaries with a total corpora count of 2 or more (52 pairs)
were arranged in ascending order of corpora count. The individual ovary
corpora counts of each pair were compared and the difference between them(d) was squared (d2) and divided by the total corpora count (N). A plot ofd 2/N on N revealed no pattern of increasing variability with corpora count,and insignificant correlation of p>0.05 with r=-0.127, d.f.=50. We may
therefore conclude that there is no indication of differential activity between
halves of ovary pairs.
Both ,Slijper (1949) and Laws (1961) found a tendency for the rightovary to be more active in fin and blue whales. Larsen (1984), examining left
and right ovaries for a small sample of Greenland minke, found no significant
bi-lateral ovarian activity.

ACKNOWLEDGEMENTS
The assistance from and collaboration with staff of the Far Seas Fisheries
Research Laboratory, Shimizu, are gratefully acknowledged; their provisionof the material and associated data have enabled this study to be carried out.
Dr Seiji Ohsumi is especially thanked for his help. The advice and criticism
from Dr John Harwood, Dr Richard M. Laws and Dr Helene Marsh in theanalysis and preparation of the paper have been useful, and they are thanked
for their help. The advice of Dr Hidehiro Kato and anonymous referees wasof great assistance.

Sci. Rep. Whales Res. Inst.,
No. 38, 1987

MINKE WHALE'S OVARY87

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EXPLANATION OF PLATE
PLATE Ia. Old corpus albicans (on right) and corpus atreticum type a (on left) showing pigmentation.
b. External appearance of a recent corpus atreticum type a, showing the stigma (siteof rupture).
c. Internal appearance of recently ruputured follicle above (Plate lb) showing commencement of luteinisation.
d. Uneven luteinisation of corpus atreticum type a, with a fibrin-filled vesicle (onleft), showing the stigma and adjacent old c. albicans (on right).
e. Corpus atreticum type a (centre) showing yellow pigmentation and the stigma(point of follicular rupture), and adjacent follicles.
f. Corpus atreticum type a with a central vesicle.

Sci. Rep. Whales Res. Inst.,
No. 38, 1987

68

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