GRAAFIAN FOLLICLES 349 



pregnancy humpback whales have, therefore, been inserted in Text-fig. 4 at a foetal length cor- 

 responding to late pregnancy in the fin whale. 



Another factor which must be mentioned is that in mature blue and fin whales the diameter of the 

 largest follicle is proportional to the body length (Nishiwaki and Oye, 195 1 ; Nishiwaki and Hayashi, 

 1950). It is shown later (p. 411) that older mature females are in advance of younger mature females 

 in their sexual cycle and, therefore, have the larger foetuses at any one time. In the material under 

 consideration, which is limited in time to a few months, the average maternal age and the average 

 maternal body length increase with increasing foetal length. In accordance with the findings of the 

 Japanese workers there would, therefore, be a bias towards larger follicle diameters in the second half 



100 



§ so 



9 

 8 



U 



uj 



i- 

 uj 5 



5 



-1 3 

 U 



6 01 ' o^ 10 20 x> 10 to ° 



PREGNANT (FOETAL LENGTH)] LACTATION [ RESTING j PREGNANT 



' J 'j/a's'o'n'd'j 'f'm' a'm'j 'Jv'a's'o'n'd' J ' f'ma'm' JJy A S 



Text-fig. 4. Follicular activity at different stages of 2-year cycle. Above, percentage of follicles over 3 cm. in diameter; 

 below, size of largest follicle (mean ± 2 S.E., and range; near-term values from humpback whale). 



of pregnancy. If allowance is made for this there is an even greater decline in follicle size during the 

 second half of pregnancy in individual females, which gives a closer agreement with the findings for 

 late-pregnancy humpback whales. 



Lactating females 



Owing to the regulations prohibiting the taking of lactating whales and the small numbers entering 

 antarctic waters, follicle records from only 56 lactating fin whales are available. The majority of these 

 females would be in late lactation (see p. 446). They show a mean maximum follicle diameter of 

 1 -93 ±0-34 cm. and a range from 0-5 to 5-0 cm. Chittleborough (1954) gives the range of maximum 

 follicle diameter for 60 humpback whales in late lactation as 1-0-5-0 cm., which is in very close 

 agreement, especially since some of the fin-whale records may be from females in mid- or early 

 lactation. (It is not possible to estimate the stage of lactation of individuals.) For three humpback 

 whales in mid-lactation the largest follicles are i-i, 1-5 and i-8 cm. and for three in early lactation the 

 maximum follicle sizes were o-6, o-8 and 2-6 cm. respectively. These were probably in very early 



