362 DISCOVERY REPORTS 



and at the base of the trabeculae. Only part of the follicle wall has contributed to the luteal tissue and 

 for the most part the follicle has undergone avascular hyaline degeneration to form a hard knob of 

 collagenous tissue. 



Other accessory corpora illustrated in Text-fig. 10, have formed in this way either from the whole 

 {c, i) or part (d, g, h,j, k, I) of the mural epithelium. Quite large accessory corpora can form without 

 ovulation; one measuring 37 mm. in diameter is illustrated in Text-fig. ioa4.This was the smallest of 

 four active corpora in the ovaries of a 76-ft. female taken on 27 February 1954, which was pregnant 

 with a 4-2 m. male foetus. There were 11 corpora albicantia. The three largest corpora lutea have 

 formed from ovulated follicles and one is conspicuously everted, with a corona measuring 7-5 x 8-o cm. 

 Another accessory corpus luteum (Text-fig. 10b) which measures 4-5 cm. has a stigma on the surface 

 of the ovary and one lobe of luteal tissue is undergoing hyaline regression. Text-fig. 10/ is an example 

 of an accessory corpus luteum, 1 -9 cm. in diameter, formed from an unruptured follicle, in which 

 the central cavity has been obliterated by an ingrowth of luteal tissue. 



These small accessory corpora formed from unruptured follicles are similar to the ' corpora lutea 

 atretica' of Brambell (1956, p. 501), but the membrana granulosa appears not to degenerate and theca 

 interna cells are distinguishable at the periphery. 



Conclusions 



In fin whales there is perhaps a slight tendency for the right ovary to ovulate more frequently than 

 the left ovary. It appears that the size of the mature graafian follicle of fin whales at ovulation is about 

 7 cm. but there is clearly a great individual variation in size at this stage. 



The corpus luteum of ovulation averages 8-28 cm. in diameter, weighs about 375 g. and probably 

 persists for 15-20 days. There is no evidence that pseudopregnancy ever occurs. If pregnancy super- 

 venes, the newly formed corpus luteum continues to grow for about 2 months, when it reaches an 

 average size of 11-44 cm - an d weighs about 881 g. There is no evidence of a decrease in size in late 

 pregnancy. No correlation between the size of the corpus luteum and the age of the female has been 

 found. Accessory corpora lutea of ovulation or pregnancy are uncommon, those above about 7 mm. 

 in diameter amounting to only 3-7% of all corpora lutea. They form either from ovulated follicles or 

 by luteinization of smaller unruptured follicles, and have a mean diameter of 3-88 cm. and an average 

 weight of about 45 g. Additional corpora lutea are not formed during pregnancy. 



No constant morphological or histological differences have been found between corpora lutea of 

 ovulation and corpora lutea of pregnancy. The incidence of vesicular corpora has been studied and gives 

 no support to Robins's (1954) suggestion that the presence or absence of a cavity is diagnostic of a corpus 

 luteum of pregnancy or a corpus luteum of ovulation respectively. In fact only 17-1 % of corpora lutea 

 of pregnancy have a fluid- or gel-filled cavity. Nor does the material support the contention of Dempsey 

 and Wislocki (1941) that the factor responsible for the formation of a cavity is a restricted blood supply 

 to the centre of the large corpus luteum of the whale. A corpus luteum 1 9 cm. in diameter possessed no 

 central cavity. There is, however, some evidence that the larger corpora lutea show more of a tendency 

 to be vesicular than the smaller corpora, but the difference is only 10% over the size range 6-13 cm. 

 It appears that at corpus sizes below about 3-4 cm. a cavity is present only in some of those accessory 

 corpora which have developed from unruptured follicles. 



The presence or absence of a cavity or, in corpora albicantia, of a central core of connective tissue, 

 cannot, therefore, be used to distinguish the corpora formed as a result of ovulation only, from those 

 representing pregnancy (see also p. 365). 



Van Lennep (1950) suggests that at the same stage of regression corpora lutea of ovulation can be 

 expected to be smaller than corpora lutea of pregnancy and this might afford a means of distinguishing 



