394 DISCOVERY REPORTS 



rough estimation that in its first year the diameter of the corpus luteum b probably 

 becomes reduced by about 40 per cent, and since this is the period during which 

 reduction is most rapid it is to be supposed that many years must pass before it could 

 completely vanish if indeed the last traces of it ever do disappear. It seems that this 

 much may be inferred, even though a quantitative estimation of the average rate of 

 regression is inadmissible. 



Further evidence of the longevity of the corpora lutea b may be obtained by com- 

 paring their numbers in the ovaries with the lengths of the whales from which they 

 were taken. This comparison is shown in Figs. 129 and 130. It will be seen that there 

 is a great diversity in the numbers of corpora lutea at any given whale length, but 

 the important fact emerges that in general the smaller whales have fewer corpora lutea 

 than the larger whales, the correlation being better defined in the smaller than in the 

 larger whales. Now up to a point the length of a whale is obviously some indication 

 of its age, and it must be supposed that the correlation existing between the number 

 of corpora lutea and the length of the whale is in fact a correlation of some kind between 

 the number of corpora lutea and the age of the whale. Female Fin whales become 

 adult when they reach a length between 19-5 and 20-5 m. (see p. 417) and one 

 would expect that they would normally continue subsequently to grow a metre or two 

 beyond this length, some ceasing to grow at about 22-0 m. others going on to 23-0 or 

 24-0 m. On the supposition that the number of corpora lutea are an indication of the 

 age of the whale this fits in well with the fact that in Fig. 130 there is a more obvious 

 correlation in the case of whales measuring 19-5 to 21-5 m. than in the case of the 

 larger whales, many of which will have ceased to grow and whose length is thus little 

 indication of their age. 



If all the corpora lutea in the ovaries were those of the previous season (as was 

 suggested by Barrett-Hamilton) this correlation with the length of the whale 

 should not exist as there is no reason why large whales should ovulate without the 

 occurrence of fertilization more times than small whales. Persistence and accumulation 

 of the corpora lutea, however, explains the correlation at once. 



It has already been pointed out that although the number of corpora lutea varies to 

 some extent with the length of the whale, there is still a great diversity in the number 

 occurring at any particular whale length. This can be set down to one of two causes. 

 The first of these is the difi^erences in length attained by the whales at and after sexual 

 maturity, and has already been discussed in certain particulars. Although female Fin 

 whales become mature mostly at about 20-0 m. and Blue whales at about 237 m. the 

 difference actually between the smallest mature and the largest immature whale is 

 relatively large. Then where for instance two whales diff"er slightly in length, the difl^er- 

 ence may be due to age, the rates of growth having been equal, or it may be due to 

 differences in the rate of growth, the ages being equal. 



The second cause for the variation in the numbers of corpora lutea is due to the 

 difTerence between the number of pregnancies and the number of ovulations which 

 may have occurred. From evidence already given it may be taken that ovulation 



