THE CORPUS LUTEUM 355 



largest follicles measured 5-3 and 3-7 cm. The fin-whale follicle size at ovulation is probably appreci- 

 ably greater than 6 cm. 



These observations suggest that in the fin whale the follicle ruptures when it enlarges to about 

 7 cm., possibly at a greater size than this, so that the mature corpus luteum of pregnancy is about one 

 and a half times the size of the follicle at ovulation. Immediately after ovulation the ruptured follicle 

 loses fluid and shrinks to about 4 cm. As the folding of the follicle wall becomes more obvious luteal 

 tissue forms from the granulosa (Text-fig. 5 a) and the centre of the ruptured follicle is often filled 

 with a translucent gel, the tertiary liquor folliculi. This central part soon fills with luteal tissue though 

 a central cavity may remain. At the same time the newly formed corpus increases in size. 



Harrison (1949, pp. 245-7) has described the histological appearance of a corpus luteum at this 

 stage of development in the pilot whale, Globicephala melaena, and concludes that the general appear- 

 ance is of an open ' lace-like ' arrangement of luteinizing granulosa cells. Considerable trabeculation is 

 present (see Text-fig. 56); projections of theca externa containing thecal vessels form the central core 

 of the trabeculae, and large groups of theca interna cells are present at the bases of the trabeculae and 

 at the periphery of the developing gland. The slit-like central cavity is lined by fibroblasts. 



In fin and humpback whales a fresh, unfixed, recently formed corpus luteum can usually be dis- 

 tinguished macroscopically from later stages. The outer membrane is very thin, with numerous blood 

 vessels visible immediately beneath it, and the luteal tissue is soft, pale in colour and expanded in 

 contrast with the firmer, more yellow tissue in later stages and especially during pregnancy. The initial 

 size of the corpus luteum is dependent on the size of the follicle at ovulation. In the recently ovulated 

 class it averages 8-28 ±0-82 cm. (range 1 "5— 13*5 cm.). Once formed it continues to increase in size 

 for some months if pregnancy supervenes. The distribution of lipoids at different stages of pregnancy 

 is illustrated in plate IV. 



Bilateral activity of the ovaries 



Slijper (1949) gives records of 17 fin whales for which it was known whether the corpus luteum of 

 pregnancy came from the left or right ovary, and there are 37 similar observations made at South 

 Georgia by the ' Discovery ' Investigations. In this sample of 54 fin whales the corpus luteum was in 

 the right ovary in 32 cases (59-2%). Similarly for blue whales, Slijper gives 22 records and there are 

 an additional 21 records from South Georgia. In 24 of these (55-8 %) the corpus luteum of pregnancy 

 was in the right ovary. Slijper's original sample indicated that in 59 % of blue whales and 65 % of fin 

 whales the corpus luteum was in the right ovary. 



As regards the distribution of corpora albicantia, there are in all records of 671 corpora albicantia 

 from 69 fin whales and 41 1 corpora albicantia from 54 blue whales which are relevant. Of these 

 51-8 and 52-3% were in the right ovary of fin whales and blue whales respectively. Combining both 

 the corpora lutea and corpora albicantia, of 724 fin-whale corpora 5 2-4 ±3 "72% were in the right ovary 

 and of 454 blue-whale corpora 52-4 + 4-68% were in the right ovary. Even when the blue and fin 

 whale data are combined the distribution is not significantly in favour of the right ovary (52-4012-92), 

 but nevertheless it seems possible that there is a slight prevalence of the right side in both these species. 



Slijper (1949) presents evidence which appears to show that in the Odontocete whales ovulation 

 occurs almost invariably from the left ovary, and he also gives a number of observations which suggest 

 that the left ovary is larger than the right. Both Slijper, and Brambell (1956) refer to the prevalence 

 of the right side in bats, and the left side in monotremes. Slijper suggests that the prevalence of a 

 particular ovary is connected with uniparity and discusses further examples. He presents one case 

 of transference of the ovum from the right ovary to the left uterine cornu in a fin whale. 



