35 o DISCOVERY REPORTS 



lactation because the corpus luteum was still large (97 mm.) and in gross appearance similar to the 

 corpus luteum of pregnancy. Of the humpback females in late lactation 18% had maturing follicles 

 (diameters 35-50 mm.) or had recently ovulated. Of the lactating fin whales 19-6% had maximum 

 follicle diameters of 35-50 mm. and five had recently ovulated (see Table 19, p. 430). Such close 

 agreement might be taken to suggest that the larger follicles in late-lactating female fin whales may be 

 approaching ovulation, but it is also possible that some follicles are regressing after a former period 

 of activity, as appears to be the case in the second half of pregnancy. Gross and histological examina- 

 tion suggests that the majority of graafian follicles in fin whales taken in the Antarctic are in atresia, 

 but that some are apparently healthy. 



An examination of follicle sizes in earlier and later phases of the sexual cycle throws some light 

 on this point. We have seen that in the second half of pregnancy follicle size was declining (expressed 

 as absolute maximum size, mean maximum size and percentage above 3 cm.), and from late pregnancy 

 to late lactation there is an increase in follicle size. The six records from early and mid-lactation 

 humpback whales are not sufficient to permit any firm conclusions, but in view of the probable 

 follicle size in late pregnancy the 2-6 cm. follicle in early lactation is suggestive of post-partum 

 follicular activity. In the fin whale, considering follicle size alone, either there is a period of follicular 

 activity at some time between parturition and late lactation, or the follicles in late lactation are maturing, 

 or both possibilities obtain. 



As regards the first alternative 15 lactating fin whale females were simultaneously pregnant (p. 430). 

 The mean foetal length is 1-69 m., which corresponds to a foetal age of about 7 months and means that 

 the current pregnancy began very soon after the termination of the previous pregnancy. It appears 

 likely that nearly all lactating females have a post-partum ovulation but that, owing to the physio- 

 logical demands of lactation, the majority fail to conceive an embryo. Evidence confirming this view 

 is presented in a later section of this paper (p. 430). 



The second alternative is discussed in the following section. The evidence of five late-lactation females 

 which had recently ovulated is highly suggestive. 



' Resting ' females 

 ' Resting ' females are here defined as mature females which are not pregnant, not lactating and not 

 recently ovulated (that is, with corpus luteum), but some of them appear to be approaching ovulation, 

 or to have ovulated fairly recently. The mean maximum follicle diameter of this group is 1 -93 ± 0-29 cm., 

 which is virtually identical with the figure for lactating females. The mean maximum follicle diameter 

 for 'resting' humpback whales is 2-0 cm. (Chittleborough, 1954) although by definition this is not 

 a directly comparable group. 



Four considerations suggest that this class of fin whales undoubtedly includes some females in 

 pro-oestrus or recent post-oestrus. First, the 'resting' condition is usually the state preceding 

 pregnancy (except for post-partum conceptions). Secondly, it embraces the ' recently ovulated ' class, 

 because ovulation succeeds dioestrus (or anoestrus) and pro-oestrus, and is in turn succeeded by 

 pregnancy, dioestrus or anoestrus. Thirdly, if we accept as the criterion of current or recent follicular 

 activity the possession of follicles over 3 cm. in diameter, then 16% are or have recently been active. 

 Fourthly, the largest follicle in this group was 8 cm. in diameter, as compared with 5 cm. in late 

 lactation. 



Evidence will be presented later in this paper (p. 436) showing that there is in fact an ovulatory 

 period at the transition from lactation to 'resting' anoestrus. This explains the range of follicle 

 diameters which is found and also the existence of a class of ' recently ovulated ' females in antarctic 

 waters during the summer months. 



