BREEDING AND GROWTH 417 



whales a pair of ovaries weighing less than 2 lb. generally indicates immaturity, the 

 corresponding figure for Blue whales being slightly higher (see Figs. 120 and 121). 



6. Condition of the mammary glands. The appearance of the mammary gland after 

 flensing, and to some extent its thickness, form a useful quick method of determining 

 whether or not a whale is mature. There are only occasionally intermediate cases where 

 the method cannot be applied and most of these can be settled by a histological examina- 

 tion of the gland. 



7. Condition of the ovarian follicles. These are not of great importance except where 

 they serve as an indication in immature whales of the approach of maturity. 



Among males there is very much less to go on, but information can be obtained from 

 the following points : 



1 . Size of the pe?iis. This is perhaps the readiest method of distinguishing mature 

 from immature males, but it can be used only to distinguish definitely mature from 

 definitely immature whales, for the growth of the penis at maturity is not conspicuously 

 sudden. Among Fin whales a penis exceeding about 1-5 m. would usually indicate 

 maturity, and about the same figure would apply to Blue whales. 



2. Size of the testis. This is more convenient than taking the weight, as there is a 

 great range in size. If the size, measured in the manner explained on p. 406, exceeds 

 about 4 in the case of Fin whales, and say 5 in the case of Blue whales, the whale is 

 generally mature (see Fig. 139). 



3. Histology of the testis. This is the most reliable method of distinguishing maturity, 

 but it is naturally laborious. The distinction between the mature and immature testis, 

 as seen in sections, is fully explained in the section on the reproductive organs. 



According to calculations based on a large number of individuals distinguished as 

 mature or immature by the above methods, the following figures may be taken as 

 accurate estimations of the mean lengths at which Blue and Fin whales become sexually 

 mature : 



Blue females ... ... 237 m. or 77 ft. 9 in. 



Blue males 22-6 m. or 74 ft. 2 in. 



Fin females 20-0 m. or 65 ft. 7 in. 



Fin males 19-5 m. or 63 ft. 8 in. 



The corresponding figures calculated by Hinton and based on the records left by 

 Barrett-Hamilton are according to these results mostly a little too low. He gives 

 75-80 ft., 70 ft., 61 ft. and 60 ft. respectively. 



The value of establishing the mean length at which maturity is reached lies in the 

 fact that it enables one to calculate the percentage of mature whales in lists of catches 

 in which only the sex and length are given. The following notes will give some idea of 

 the degree of accuracy with which maturity can be gauged by the length alone : 



Blue females. Of 402 female Blue whales there were three clearly mature measuring 

 less than 237 m. and three clearly immature measuring more than 237 m. The smallest 



