MULTIPAROUS FEMALES 433 



Moreover, if the species were polyoestrous, then in pregnant females the corpus albicans derived 

 from the corpus luteum of the previous pregnancy, in which regression should now be proceeding 

 at a much slower rate than at the beginning of regression, would then appear to have undergone much 

 greater shrinkage than a young corpus albicans from the first of the hypothetical two recent dioestrous 

 cycles. In fact the ovulation which preceded that which led to the current pregnancy almost certainly 

 occurred at the beginning of the resting period, and is probably represented by the second largest 

 corpus albicans in pregnant females (see below). 



Now, if there is no post-partum ovulation in the majority of females and if those which do have 

 a post-partum heat usually become pregnant as a result, the second largest corpus albicans of non- 

 pregnant, lactating females should represent the further regression of the largest corpus albicans in 

 pregnant females. This also gives an acceptable rate of regression. 



The third largest corpus albicans of lactating females should then be derived from the third largest 

 corpus of pregnant females (that is, the second largest corpus albicans, since the largest corpus is the 

 corpus luteum). It will be apparent from the figure that this is unlikely. The mean size of the third 

 largest corpus albicans of lactating females is actually about i mm. greater than that of the third largest 

 corpus of pregnant females and, taking into account the 95 % confidence limits, the maximum probable 

 rate of regression would be less than 2 mm. over a period of about 1 1 months. This is much too slow, 

 and we must conclude that the third largest corpus albicans in lactating females is the product of 

 the regression of an ovulation in the period between parturition and the latter part of the lactation 

 period. As we know that lactating females which are pregnant must have conceived at a post-partum 

 ovulation, it is probable that this third largest corpus albicans of non-pregnant lactating females also 

 represents a post-partum ovulation. In seven months it is presumed to have decreased in diameter by 

 50% (on average from 8-28 to 4-15 cm.) and in volume by about 87% (Text-fig. 8). This compares 

 with a decrease in the diameter of the regressing corpus luteum of pregnancy of 5 1 % ( 1 1 -44 to 5 -58 cm.) 

 and a decrease in volume of about 88%, over a slightly longer period. This close agreement between 

 observed and inferred rates of regression strongly supports the conclusion that post-partum oestrus 

 is a regular feature in the sexual cycle of the majority, if not of all multiparous fin whale females. 



The fourth largest corpus albicans of lactating females is, therefore, likely to be derived from the 

 second largest corpus albicans of pregnant females, and this gives a reasonable amount of shrinkage 

 (about 7 mm. in diameter), associated with regression over about 11 months. 



Let us now consider the resting females. Although this anticipates the next part of this paper, it is 

 more convenient to complete this discussion of the regression of corpora before turning to other points. 

 It has been shown that the largest corpus albicans in lactating females represents the former corpus 

 luteum of pregnancy, and it has been suggested that this is also the derivation of the largest corpus 

 in resting females. In the next section it is shown that the group of resting females includes some 

 which have recently ovulated, the ovaries containing an active corpus luteum, or a body partly 

 resembling a corpus luteum and partly a corpus albicans ; and others with a very large recent corpus 

 albicans. In this group the largest corpus albicans in some individuals will represent the former corpus 

 luteum, with a mean size slightly less than the largest corpus albicans in lactating females ; in other 

 ' resting ' individuals the largest corpus albicans will be the product of a recent post-lactation ovulation. 

 Similarly, the size frequency distribution of the second largest corpus albicans in resting females will 

 include some which represent the previous corpus luteum of pregnancy, and some which are derived 

 from a recent ovulation. The effect of this should be to make the largest corpus albicans larger, and the 

 second largest corpus albicans smaller, than they should be when regression of the recently formed 

 corpus albicans is more advanced. In pregnant females there is no recently formed corpus albicans, 

 and the expected separation between the second and third largest corpora albicantia is found. 



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