4i8 DISCOVERY REPORTS 



females which had ovulated more than once the diameters of the largest corpora albicantia were 

 4*5 > 3 '3 an d 3 cm - respectively, suggesting that there had been quite a long interval between the last 

 two ovulations and that they do not represent successive ovulations in a polyoestrous ovulatory cycle. 



The estimated pairing season for the ' o corpus albicans ' group is April-November (inclusive) 

 (Text-fig. 37). The present material shows that puberty can be attained and the first ovulation take 

 place several months before or after the normal pairing season for this group. March is the only 

 month for which there is no record or estimate of a female fin whale reaching puberty. 



For six of these females observations on the extent of the diatom film were made. In three of them 

 there was no diatom infection (two in January, one in February); in one (November) it was incipient; 

 in one moderate (November); and in one it was heavy (December). The latter record relates to the 

 female in which the recent corpus luteum had just begun to regress. As the diatom film is acquired in 

 antarctic waters these limited data suggest that the corpora lutea of ovulation in these whales formed 

 during or just after the southern migration. 



Text-fig. 40. a, Size of largest corpus albicans of 14 nulliparous females without a corpus luteum; antarctic material. (Black 

 circles, only corpus albicans; white circles, largest of two or more corpora albicantia.) b, Monthly frequency of ovulation of 

 41 pubertal females taken in the Antarctic. 



Certain other evidence supports this hypothesis. There are records of the size of the largest corpus 

 albicans in a number of pubertal nulliparous females taken in the Antarctic. In Text-fig. 40, a, the 

 diameters of 14 such corpora (from ovaries in which there are no active corpora lutea) are plotted 

 according to the date of collection. They show an apparent decline in size, probably representing 

 the shrinkage associated with regression. The sizes of these corpora suggest that most of them were 

 derived from corpora lutea of ovulation which formed as late as November, December, or January. 

 The ovaries of these females do not contain follicles undergoing pro-oestrus-enlargement and this 

 confirms that these corpora do not represent the first ovulation in a series of dioestrous cycles. 



If we now examine the entire group of pubertal nulliparous females (which incorporates animals 

 not included in the discussion above because there were no records of corpora size for them) we can 

 obtain a better estimate of the time of ovulation. This material comes from each of the 6 months 

 November-April and for each month the percentage of animals in this group which have an apparently 

 active corpus luteum of ovulation in the ovaries (as opposed to those in which the ovaries contain one 

 or more corpora albicantia) has been calculated. This is shown in Table 16 and graphically in Text- 

 fig. 40, b. A single record for 29 December is the animal mentioned above in which the corpus luteum 

 had only just begun to regress and had characteristics of both corpus luteum and corpus albicans. 



