380 DISCOVERY REPORTS 



size of corpora albicantia of similar ages should be greater in older whales. Dr Mackintosh has shown 

 me some unpublished figures he worked out for the average sizes of corpora albicantia, based on a 

 large number of measurements. He was able to show that the average size of the nth. largest corpus 

 albicans does in fact increase with age, that is, with increasing corpora number. For example, when 

 there are 10 corpora albicantia the tenth largest is smaller than the tenth largest when there are 

 20 corpora albicantia. This finding has been inexplicable up to now, because the most obvious inter- 

 pretation, that the size of the corpus luteum and therefore of its products increases with age, is known 

 to be inapplicable (see p. 357). I am indebted to Dr Mackintosh for enabling me to refer to this work. 



It appears then that this apparent increase in the rate of ovulation is due to the lengthening of the 

 period occupied by the regression of the corpus albicans in older whales. A correction has, therefore, 

 been made, by assuming that ' old ' corpora albicantia accumulate at the same rate as ' young ' corpora 

 albicantia form. This assumption is valid if the duration of the regression period is constant and there 

 is no human error in the counts of recent corpora, and no increase in the rate of ovulation. The last 

 qualification is not quite correct, but may be ignored for the moment. 



A corrected regression line of slope 1 -o was, therefore, drawn through the intersection point of the 

 two curves shown in Text-fig. 21, giving an intersect on the x-axis of 4*45, and an estimate for the 

 average annual increment of corpora of 1-48. This figure is likely to be a little high because it does not 

 allow for any increase in the regression period while the first nine corpora are accumulating; nor are 

 we justified in assuming that there is no increase in the ovulation rate with age. There is in fact a 

 slight rise in the incidence of multiple ovulations. Taking into account all the evidence the best 

 estimate of the average annual rate of accumulation is between 1-4 and 1-5. 



Taking the duration of the ' young ' and ' medium ' stages in the regression of corpora to be about 

 1 and 2 years respectively, the mean sizes of the corpus luteum, ' young ', ' medium ' and ' old ' corpora 

 albicantia, may be plotted on semi-logarithmic paper. For both diameter and weight the initial 

 regression is very rapid, corresponding to the change from glandular tissue to collagen, and is suc- 

 ceeded by slower regression which is apparently exponential (that is to say, the points fall on a straight 

 line) until the fully regressed stage is reached. 



Corpora aberrantia 



In addition to the types of normal corpora albicantia and corpora lutea, which have been described 

 and shown to represent stages in the regression of ovulation and pregnancy corpora lutea, a very small 

 number of other corpora are present in fin-whale ovaries. 



During the examination of the 1953/54 ovaries four unusual types of pigmented bodies were identi- 

 fied. One very distinct type, small and bright orange-yellow in colour, is more properly a corpus 

 atreticum and is not considered to represent a former corpus luteum. The other three main types 

 have yellow or buff pigmentation. They were termed corpora aberrantia and distinguished by the 

 adjectives ' yellow ', ' buff-cellular ' and ' yellow and white '. In the routine examinations of part of the 

 1955/56 collection further observations were made on the incidence of these types. Out of a total of 

 2655 corpora albicantia and corpora aberrantia (corpora atretica excluded) only 37 (or 1-4%) were 

 corpora aberrantia. The 'yellow' type comprises 0-7%, 'buff-cellular' o-6% and 'yellow and white' 

 o-i%. They were present in 34 out of 273 pairs of ovaries (12-5%). 



All appear to have developed from ruptured follicles (Text-fig. 236 and c), but it is difficult to 

 establish the cause of their formation and their fate is uncertain. 



