376 DISCOVERY REPORTS 



Thus, there is no permanent record in the form of fully regressed corpora, of 4-5 % of corpora lutea 

 of ovulation and pregnancy in the ovaries. On the other hand, 2-1% of the 'old' corpora albicantia 

 are expected to represent accessory corpora lutea, so there is a net loss of 2-4%. So far we have been 

 considering fully regressed corpora, but 42-5 % of all corpora albicantia in this sample of 4065 were 

 either 'young' (15-5%) or 'medium' (27-0%) and had not fully regressed. The corpus luteum which 

 would eventually be represented by a fully regressed corpus albicans of 7 mm., can be shown in these 

 two groups to have regressed to 1-4 or i-o cm. respectively, which means that the effective number of 

 lost corpora albicantia considered as a percentage of all corpora albicantia (' young ', ' medium ' and 

 'old') is considerably less than 2% and probably nearer 1%. 



This is such a small fraction of the total that it is probably within the observational error and it will 

 be assumed for the purposes of applying the corpora counts to age-determination that all ovulations, 

 whether successful or not, are recorded permanently in the ovaries by corpora albicantia. 



Accumulation of corpora albicantia 

 It has now been established that corpora albicantia of the three groups, ' young ', ' medium ' and ' old ' 

 represent stages of regression. It might be possible to obtain an estimate of the absolute rate of 

 regression by examining the proportions in which they occur and the relative rates at which they 

 accumulate. 



For 393 pairs of ovaries with three or more corpora albicantia collected in the season 1955/56 the 

 mean numbers of 'young' corpora albicantia for pregnant and non-pregnant females are 1-522 ±0-125 

 and 1 -586 ±0-185 respectively. There is no significant difference between these values and the com- 

 bined mean value is i-544±o-i04. Individual females had up to five 'young' corpora in the ovaries; 

 only 15 out of 393 had more than three. Unfortunately, there are only nine records of lactating 

 females, and they give a mean number of 1 -778 ± 0-688 ' young' corpora albicantia. This is not signifi- 

 cantly different from other samples, but the large standard error means that there could be a difference 

 of up to one 'young' corpus albicans between lactating and non-lactating females. 



At the time of writing the 1956/57 sample has not yet been fully examined, but for 251 pairs of 

 ovaries which have been sliced (and have three or more corpora albicantia) the mean numbers of 

 'young' corpora albicantia for pregnant and non-pregnant females are 1-473 ±0-126 an ^ J '545 ±0-3 18 

 respectively. There is thus no significant difference between the mean numbers of ' young ' corpora 

 albicantia present in each of these two seasons. 



Using the 1955/56 data it is also possible to compare the mean number of ' young ' corpora albicantia 

 of females in part of the former sanctuary (area I), 1 with the mean number of ' young ' corpora in the 

 older whaling areas (material mainly from area II). In area I the mean is 1-674 ±0-200 an d in the 

 other areas it is i-507±o-i2i. Again the difference is not significant. The area I stock has been only 

 slightly fished and the annual mortality rate appears to have been about 10% prior to 1956, whereas 

 it has been found that in the older whaling areas the mortality rate is possibly over twice as high 

 (International Commission on Whaling, Eighth Annual Report, p. 24). 



It would appear then, that there is little change in the average number of ' young ' corpora albicantia 

 produced from year to year, and that even great changes in the condition of the stock have had no 

 significant effect on the mean number of ' young ' corpora in the ovaries. This is a point of con- 

 siderable importance which will be referred to later (p. 384) in connexion with the constancy of the 

 average annual increment of corpora albicantia. 



Considering now those ovaries in the 1955/56 collection containing five or more corpora albicantia 

 (sample size 323) we can determine the mean number of 'medium' corpora present. This is 3*220± 



1 There are six antarctic whaling 'areas'. Area I is in the South-east Pacific Ocean (60-120° W.). 





