384 DISCOVERY REPORTS 



Some evidence bearing on the rate of accumulation of corpora albicantia is provided by examination 

 of the absolute and relative numbers of these three types of corpora albicantia. There are no significant 

 differences between the mean numbers of ' young ' corpora albicantia in pregnant and non-pregnant 

 females, nor between samples taken in consecutive years, the mean value being 1-54. Large variations 

 in the mortality rates of populations of fin whales apparently have no significant effect on the rate of 

 formation of corpora albicantia. It follows that if there are differences in the rate of accumulation 

 they must be slight. The mean number of 'medium' corpora albicantia is 3-22, giving a ratio of 

 ' young ' to ' medium ' corpora of 1 : 2. The average duration of the ' medium ' stage is probably about 

 twice as long as that of the ' young ' stage and it is concluded that the ' young ' corpora probably 

 represent one year's increment, and on average take about 3 years to regress to ' old ' corpora, which 

 after the first few years accumulate at the same rate as the ' young ' corpora. The mean number of 

 ' young ' and ' medium ' corpora apparently increases with increasing total corpora number. Evidence 

 is presented which strongly suggests that this is the result of a progressive retardation in the regression 

 of corpora with increasing age of the female. Taking this into account the corrected estimate of the 

 mean annual increment of corpora albicantia is between 1-4 and 1-5 although the possibility that 

 this is in error by a factor of 2 cannot be excluded. 



The possibility of gross morphological differences between corpora albicantia derived from corpora 

 lutea of pregnancy and ovulation respectively, as suggested by Robins (1954) and Van Lennep (1950) 

 has been examined and rejected. Peters (1939) claimed to have found such a macroscopically 

 recognizable difference, based on differences in the colour and texture of the gland, in the arrange- 

 ment of the connective tissue and the trabeculae. Although he examined 27 pairs of blue-whale ovaries 

 and 57 pairs of fin-whale ovaries, with altogether 500 corpora, his counts establishing the proportions 

 of the two groups of corpora are based on only 4 pairs of blue-whale ovaries (with 75 corpora in all) 

 and 7 pairs of fin-whale ovaries (with altogether 97 corpora). He states that the darker group repre- 

 sent corpora of pregnancy and the lighter-coloured are corpora of the cycle. Since every corpus of 

 pregnancy corresponds to a breeding period, and, therefore, on average to 2 years of life, the average 

 number of ovulations per cycle can be obtained from the ratio of pregnancy corpora to ovulation 

 corpora. For instance, if this ratio is 1 : 1 then there will be two ovulations per cycle. Peters's actual 

 values for blue and fin whales are respectively 1 -9 and 1 -8 ovulations in 2 years. 



Unfortunately Peters did not give precise details of these differences in his paper, but promised 

 that they would be given in a later paper which was not completed before his death. No macroscopic 

 difference attributable to different origin has been observed in the present material although several 

 thousand corpora have been examined in detail. The only clear distinction which has emerged is that 

 which is the result of age changes in the corpora. The differences between recent ('young' and 

 'medium') and 'old' corpora are similar to those indicated by Peters as distinguishing corpora of 

 pregnancy and ovulation respectively. 



It is instructive to assume that recent and ' old ' corpora are derived from corpora lutea of pregnancy 

 and ovulation respectively and see what figure is obtained for the rate of ovulation. The mean number 

 of corpora albicantia per female in the 1953/54 an< ^ I 955/5° samples is 9-7. From Text-fig. 21 the 

 mean numbers of recent and 'old' corpora albicantia at this corpora number are 4-6 and 5-1 respec- 

 tively, giving a ratio of i:i-i and, according to Peters's hypothesis, an assumed 2-1 ovulations per 

 2-year cycle. The agreement with Peters's estimate of i-8 is rather close when it is recalled that Peters's 

 sample only contained seven whales and would have a correspondingly large variance. It is the author's 

 opinion that this is probably the basis of Peters's estimate and this assumption, that recent and ' old ' 

 corpora are of different origin, is known to be incorrect. 



It may be possible to identify corpora albicantia derived from corpora lutea of ovulation and 

 pregnancy by quantitative histological examination. The rate of regression of corpora albicantia 



