GRAAFIAN FOLLICLES 347 



or above 3 cm. in diameter are maturing and approaching the first ovulation. The fin- whale material 

 shows that in most mature females and in some immature females, follicles above 3 cm. may be in 

 cystic atresia. The present evidence suggests that immature fin whale females may be approaching 

 the first ovulation in June, July, August, October, November and possibly September and December. 



50 



§ 40 



2 

 < 



a 



20 14 



SIZE OF SAMPLE 

 3 4 



20 14 



30 



2 20- 



2 



< 10 



5 



J J A 



MONTHS 



Text-fig. 3. Monthly mean size of largest follicle in 62 pairs of ovaries from immature females which 

 were approaching puberty (mean ±2 S.E. ; black, South Africa; white, Antarctic). 



Adult ovaries 



Primary follicles 



Laws (1957) stated that in adult fin-whale ovaries the tunica albuginea is from 0-95 to i-6 mm. in 

 thickness and that no primary follicles had been seen in histological preparations. Since then very 

 small numbers of primary follicles have been observed in the ovaries of two mature fin whales, one 

 of length 73 ft. in anoestrus, and one of 70 ft. pregnant with a foetus 279 m. long. These primary 

 follicles are very few and scattered in distribution and their absence in other mature females examined 

 is probably more apparent than real. For instance, the average surface area of one ovary of a non- 

 pregnant mature female is about 800 sq. cm. and it would be a major task to sample even one pair 

 adequately. Usually only one or two samples representing 0-1-0-3 % of the surface area are embedded 

 and in a large series it is impracticable to examine more than about one-tenth of this material histo- 

 logically, so that less than 0-05 % of the cortex is searched. The large size of the organ is a very real 

 handicap in any studies of the finer details. It is probable, however, that the numbers of primary 

 follicles decrease greatly with age. In the rabbit, for instance, Desaive (1941) found that these 

 decreased from 120,000 at 3 months to 6000 at 18 months. 



' Recently ovulated ' females 

 There are records of the diameter of the largest follicle in 23 recently ovulated fin whales taken in the 

 Antarctic, most frequently in November-December. These may be characterized as females which had 

 a large and apparently normally active corpus luteum in the ovaries, but in which an intensive search 

 of the uterus failed to reveal a foetus. Four pregnant females with a foetus less than 4 cm. long are 

 also included in this group. The correctness of the diagnoses of recent ovulations is confirmed by the 

 data set out in Text-fig. 4. Thus, in pregnant females considered as a whole, the mean maximum 



