434 DISCOVERY REPORTS 



The second largest corpus albicans in resting females, therefore, represents a recent ovulation at 

 the beginning of the resting period (see below, p. 436, for other evidence of this ovulatory period), 

 which should have a mean diameter larger by probably about 1-2 mm. (The largest corpus albicans 

 in resting females appears to be approximately 1-2 mm. larger than expected by comparison with the 

 largest corpus in lactating females.) The third and fourth largest corpora are then seen to be derived 

 from the second and third largest corpora of lactating females, and become in turn the third and 

 fourth largest corpora albicantia of pregnant females, when they again show a relatively close size 

 grouping as do the second and third corpora of lactating females. Only in the fourth largest corpus 

 albicans in resting females has the line indicating regression not been drawn to the mean, but it is 

 within the 95 % confidence limits. In all other corpora groups the suggested regression lines are drawn 

 through the means, and give a consistent picture of rates of regression in size. 



The second largest corpus albicans of pregnant females is now seen to be derived from the second 

 largest corpus albicans in resting females. If this resulted from an ovulation at the beginning of the 

 resting period, then it has decreased in size from 8-28 cm. in diameter to 4-05 cm. in about a year 

 (from the beginning of the resting period to the eighth month of pregnancy), that is by 5 1 % . This is 

 not in close agreement with the estimated average percentage shrinkage of corpus luteum of pregnancy 

 (51%), and post-partum corpus luteum of ovulation (50%), over a shorter period of time. It is 

 possible that the size regression of corpora albicantia is slower during pregnancy than during the 

 lactation or resting phases of the cycle. This is borne out by the regression in size of the largest corpus 

 albicans in primiparous females (which will represent an ovulation). When plotted according to the 

 time-scale established for foetal length, this declines from 5-3 cm. at the beginning of pregnancy to 

 about 4-1 cm. at term, and would explain the apparent discrepancy noted above. 



A further point to be made is the relation of the fourth largest corpus albicans in resting females to 

 corpora albicantia in other groups. If we assume, for the purpose of the argument, that there is no 

 post-lactation ovulation in the fin whale, then the second and third corpora albicantia of resting females 

 should be derived from the second and third corpora albicantia of lactating females. This means that 

 the fourth largest corpus albicans of resting females does not fit into the pattern nearly so well ; its 

 mean diameter is 3-62 cm., larger than the size of the fourth corpus albicans of lactating females 

 (3-36 cm.), from which it should be derived according to this hypothesis. Together with the inde- 

 pendent evidence to be described below (p. 436) this indicates that a post-lactation ovulation is a 

 regular feature of the fin whale cycle. 



Anomalous corpora albicantia of lactating females 

 Four lactating females were examined in 1953/54. Of these, two were simultaneously pregnant, and 

 one (no. 17 17) was primiparous with one corpus albicans representing the previous corpus luteum 

 of pregnancy. The fourth (no. 636), which was the only normal multiparous lactating female examined, 

 had 15 corpora albicantia in the ovaries, the largest of which was of the usual type, measuring 3-7 cm. 

 in mean diameter, and probably represented the previous corpus luteum of pregnancy. It was similar 

 in appearance to the single corpus albicans in no. 17 17, and to the corpora albicantia in other primi- 

 parous lactating females examined later. There was one anomalous corpus albicans in the ovaries of 

 no. 636, of a type which had not previously been observed in non-lactating females (Text-fig. 466). 

 This measured 3-4 cm. in mean diameter. 



Subsequently, a further 18 non-pregnant lactating females and two pregnant lactating females were 

 examined. The corpora albicantia in the four pregnant lactating females were all of the normal type, 

 but of the 18 ' lactating only ' females (excluding two primiparous females with only one corpus albicans 

 of the normal type) nine, or 50%, possessed one of the anomalous corpora. 



