372 DISCOVERY REPORTS 



further. Those corpora included in the 'old' group are, therefore, assumed to be fully regressed 

 corpora, at least as regards size. 



Clearly there is in general no complete regression of corpora in respect of size. Nor is there any 

 evidence that the corpora shrink to a mean diameter of 2 cm. and a weight of about 5 g. and then 

 become indistinguishable from the ovarian stroma by further loss of pigment. Corpora albicantia of 

 this size can easily be identified even if they appear to have lost all their pigmentation, because their 

 white avascular connective tissue stands out against the darker colour of the ovarian cortex. Nor is 

 there any histological evidence for eventual disintegration and resorption by phagocytes, because no 

 intermediate conditions have been observed. A consideration of the rate of formation of ' old ' corpora 

 albicantia lends further support to this conclusion (Text-fig. 22). Once they begin to form the increase 

 in their number is linear showing that they accumulate at a constant rate which shows no sign of 

 decreasing, at least up to a total corpora number of 30 or 40. If their life-span was less than the period 

 covered by the material then their numbers might be expected to approach or to reach an upper limit 

 at which loss was balanced by replacement. 



It has already been established that the mean diameter of the fin-whale corpus luteum of pregnancy 

 is 11-44 cm - (P- 356), the mean diameter of the corpus luteum of ovulation is 8-28 cm. (p. 356) and 

 the mean diameter of the accessory corpora lutea is 3-88 cm. (p. 361). The latter group comprise only 

 37% of all corpora lutea examined, and their effect on the ensuing discussion is negligible. The corpora 

 albicantia, which represent former corpora lutea of all three types, have been shown to regress to a 

 mean size of 2-01 cm. 



Table 4. Mean diameter of corpus luteum and mean or modal diameter of fully 

 regressed corpus albicans for five species 



Mean or modal size (cm.) 



The frequency distribution of the diameters of corpora lutea of pregnancy is a symmetrical curve 

 with the mean approximating to the mode and it is likely that the curve for corpora lutea of ovulation 

 would be similarly symmetrical if there were more records, although the size range is greater. 

 Unfortunately, we have few records of corpora lutea of ovulation, because they persist for such a short 

 period and are mostly produced during the months before the present antarctic whaling season begins 

 (see p. 437). If the numbers of corpora lutea of pregnancy produced are equal to the numbers of 

 corpora lutea of ovulation, then a size frequency curve for all corpora lutea would have a mean which 

 would be the resultant of the mean diameters of the corpora lutea of ovulation and pregnancy, in this 

 hypothetical case 9-86 cm., which means that corpora lutea as a whole regress to 2 cm., that is by 

 79-7% of their initial diameter. On the other hand, if on average only one out of three ovulations is 

 followed by pregnancy the resultant mean size of all corpora lutea will be 9-33 cm. and the percentage 

 regression will be 78-6% of the initial diameter. It should be noted that the difference between the 

 amount of regression in these two cases is only 1 % and even with a ratio of four unsuccessful ovula- 

 tions to one pregnancy the variation is only increased to 1-7%. Conversely, if all ovulations were 

 successful and followed by pregnancy the percentage regression would be 82-6%. The range, from 

 a ratio of four unsuccessful ovulations to one pregnancy, to no unsuccessful ovulations, is only 4-6%. 



