354 DISCOVERY REPORTS 



Chittleborough gives a mean diameter for the corpus luteum of 29 females in late pregnancy, when 

 the corpus is at its maximum size, of 12-3 cm. If nine records of early and mid-pregnancy humpback 

 females (Matthews, 1937) are combined with his data the mean diameter is n-89± 1-58 cm. which is 

 virtually the same as the size of the fin-whale corpus luteum, mainly from early and mid-pregnancy 

 (1 i-44±o-54 cm.). We may, therefore, assume that the details of ovulation in the fin whale, for which 

 we have no direct evidence, are very similar to the condition in the humpback whale. 



Text-fig. 5. Morphological variation in corpora lutea. See text for explanation. 



In the humpback material there were 35 females which were found to have just ovulated, the blood- 

 stained hole (from 4 to 13 mm. in diameter) being immediately obvious on the surface of the ovary. 

 Immediately after ovulation the follicle was collapsed and the wall wrinkled. The size range at this 

 stage was 2-2-6-0 cm. (median 3-7 cm.) and the smallest recently formed corpora lutea are about 4 cm. 

 in diameter (Chittleborough, 1954, fig. 4), suggesting that the size of the original mature follicle at 

 ovulation was of the order of 6-8 cm. (see Harrison, 1948, p. 247). This is in agreement with the 

 maximum size of unruptured mature follicles given by Chittleborough ; of five above 6 cm. the mean 

 diameter was 7-1 cm., but four of these lay between 6 and 7 cm. and one was 10-5 cm. in diameter. 



In 25 recently ovulated fin whales the largest follicle (which was presumably the second largest at 

 ovulation) had a mean diameter of 3 -8 ±0-53 cm. and the absolute maximum size was 9 cm. Chittle- 

 borough (1954, p. 58) gave one example of a female humpback whale approaching ovulation; the two 



