432 DISCOVERY REPORTS 



similar age, those derived from corpora lutea of pregnancy should be larger on average than those 



which are products of the regression of corpora lutea of ovulation. 



Let us now attempt to trace the regression of corpora albicantia through successive phases of the 

 sexual cycle. The lines joining corpora in Text-fig. 45 present the conclusions; corpora known or 

 presumed to be derived from pregnancy corpora lutea are shown as black rectangles (shaded in the 

 case of resting females), and those presumed to represent ovulations are shown as white rectangles. 

 The evidence on which these conclusions are based is as follows. 



120 



no 



60 



2 



o 



a: 



UJ 



H 



UJ 



2 

 < 



so 



40 



30 



20 



R P 



STAGE OF CYCLE 



Text-fig. 45. Mean diameters + 2 S.E. of corpus luteum, and six largest corpora albicantia of pregnant, lactating 1 , 

 and ' resting ' females plotted to a time scale so as to represent two sexual cycles. See text for explanation. 



The mean size of the largest corpus albicans in lactating females is 5-58 cm.; the mean size of the 

 largest corpus albicans in lactating females which are also pregnant (not given in Table 20) is 5-61 cm., 

 and in ' resting ' females it is 5-57 cm. These mean sizes are not significantly different, and if we suppose 

 that there has been not more than one post-partum ovulation in lactating pregnant females, then 

 their largest corpus albicans must represent the corpus luteum of the previous pregnancy. The 

 largest corpus albicans in non-pregnant lactating females and resting females is presumed (because 

 of its closely similar size) also to represent the previous corpus luteum of pregnancy. 



In pregnant females the largest corpus albicans must either represent the corpus luteum of the 

 previous pregnancy, or an unsuccessful ovulation before the current pregnancy. The latter case is 

 unlikely, because it would mean that the second largest corpus albicans of pregnant females represents 

 the largest corpus albicans of resting females, and has undergone rapid regression. It would also 

 mean that the fin whale is polyoestrous, with on average two ovulations in the breeding season, the 

 second of which leads to pregnancy, whereas the evidence points to a seasonally monoestrous cycle. 



1 The lactating females are in late lactation (p. 445), so close to the resting stage. 



