396 DISCOVERY REPORTS 



can take place without fertilization, and that many follicles are ready for subsequent 

 ovulation should fertilization not occur. Now if fertilization occurs at the first ovulation 

 every season the number of corpora lutea represents the number of pregnancies which 

 have taken place and the number of years since sexual maturity was reached (except 

 possibly in the case of the oldest whales in which the oldest corpora lutea might 

 have finally vanished). With ovulation, however, taking place spontaneously and no 

 method of distinguishing the corpora lutea formed from those of pregnancy it is 

 evident that there will be great diff'erences in the numbers of corpora lutea present 

 among whales of similar length, differences depending on the number of dioestrous 

 cycles which may have taken place each season before pregnancy supervened. 



Finally, a few words may be said in recapitulation of the more important facts 

 which emerge from the study of the ovaries. There are two specially characteristic 

 features of the ovaries of the Balaenopteridae. These are the abundance of ripening 

 follicles which are so often present and the curious longevity of the old corpora lutea. 

 The number of follicles implies a capacity for producing numerous ova in quick succes- 

 sion, and this favours the supposition that these whales are polyoestrous. The suggestion 

 is further supported by other evidence. There are several cases for instance in which 

 over thirty old corpora lutea have been counted in a single pair of ovaries, and if 

 whales are monoestrous it follows that some of these have persisted for not less than 

 thirty years. It seems much more reasonable to suppose that the large number is the 

 result of several unsuccessful ovulations in a comparatively few seasons. Again, it 

 will be shown later that the breeding season is a protracted period covering several 

 months. This in itself suggests that a succession of dioestrous cycles may occur and 

 that in some cases conception occurs after the first ovulation and in other cases not 

 until several ovulations have occurred and the season is well advanced. 



The longevity of the corpora lutea provides a useful indication of the history of the 

 whale in which they occur. The weak point here, however, lies in the fact that the 

 number of corpora lutea depends partly on the number of years which have elapsed 

 since sexual maturity and partly on the number of ovulations which have occurred in 

 each sexual season, and there is no means of knowing how much each factor has 

 contributed to the number of corpora lutea which are found. However, one would not 

 suppose that more than a very few ovulations would occur before pregnancy super- 

 vened among animals living in a state of nature, even though it is likely that ovulation 

 may occasionally take place at times of year outside the season at which breeding 

 activities become general. It follows from this that a whale having twenty or thirty 

 corpora lutea b in the ovaries can hardly have been adult for less than five or six 

 years, and has more probably been adult for say twelve or fifteen years. On the other 

 hand, a whale having only three or four corpora lutea will probably be not more than 

 two or three years old. 



