BREEDING, GROWTH AND AGE 221 



maturity is reached when about fifteen corpora lutea have accumulated in the ovaries. 

 The correlation was very remarkable, for 'of 105 whales with less than fifteen corpora 

 lutea only two, with eleven and twelve respectively, were physically mature; while of 

 sixty-six whales with more than fifteen corpora lutea only four, with sixteen, sixteen, 

 twenty and twenty-one, were physically immature'. Laurie (1937, p. 236) found a 

 similar correlation in Blue whales, nearly every whale having more than eleven corpora 

 lutea being physically mature and nearly every whale having less than eleven being 

 physically immature. This accumulation of such a regular number of corpora lutea at 

 such a definite landmark as physical maturity seems to leave no doubt not only that the 

 corpora lutea persist up to and well beyond the age at which physical maturity is reached 

 but also that the accumulation takes place at a fairly steady rate. It can also perhaps be 

 argued that the ossification of the vertebral epiphyses and the accumulation of corpora 

 lutea could scarcely keep in step with one another in this way except in their relation 

 to the age of the whale, and that therefore the females normally become physically 

 mature at a fixed age in either species after the attainment of sexual maturity. 



For these reasons there seems no doubt that the corpora lutea persist for a long time, 

 and if they last thus from the first ovulation until beyond the point at which physical 

 maturity is reached there seems no reason why they should ever disappear, even in the 

 oldest whales. There is perhaps no direct proof that they do last throughout the life of 

 the animal, but Bertram (1940, pp. 61-5) concludes that they do so in the parallel case 

 of the Weddell seal on the ground of their frequency distribution. His argument, in 

 essence, is that if all the old corpora lutea remain permanently visible and countable in 

 the ovaries it would be expected that there would be progressively fewer seals possessing 

 each number of them as the scale is ascended, whereas if the corpora lutea diminish and 

 disappear after a certain interval, all seals over a certain age would tend to have the same 

 number of corpora lutea, and that a certain number near the maximum would therefore 

 occur more frequently than the lower numbers. He found, in fact, that the larger 

 numbers became progressively scarcer and concluded that the corpora lutea persist 

 throughout the seal's life. The same argument can be applied to Blue and Fin whales, 

 for Laurie's frequency curves for Blue whales (1937, p. 254) and Wheeler's for Fin 

 whales (1930, p. 417) show a gradual tailing off towards the higher numbers. 



BREEPING 

 Inspection of the ovaries and testes, and analyses of records of foetal lengths at dif- 

 ferent times of year show clearly that there is a breeding season in the southern winter 

 and that the period of gestation is nearly a year. This has been fully discussed in 

 previous publications, and all that need be said here is that the birth of the calves and 

 pairing take place over a protracted period in the southern winter. Mackintosh and 

 Wheeler (1929, pp. 426, 429) estimated that in Blue and Fin whales the height of the 

 pairing season is in June and July, and that Blue whales are mostly born in April and 

 May, and Fin whales probably a little later. These estimates were based on limited data, 

 but are not likely to be far wrong. Matthews (1937, 1938c) estimated that in the Hump- 



