426 DISCOVERY REPORTS 



height of the mean curve of foetal growth shows that the period of gestation is, on the 

 average, well started by the end of August. 



Taking the data as a whole, it may be inferred that the height of the pairing season 

 falls in both species towards the end of June. As already mentioned, the early activity 

 of the testes does not necessarily mean that pairing takes place equally early, and the 

 instances of ovulation point to the end rather than the beginning of June, for it must be 

 remembered that, as whales appear to be polyoestrous, several ovulations may occur 

 before impregnation, but none will occur afterwards. 



It will be seen that the curve for both species starts slowly but gradually increases 

 throughout pregnancy. It cannot perhaps be regarded as certain that the rate of growth 

 increases steadily during the latter part of gestation, but it is quite certain that growth 



Fig. 147. Curve of frequency of pairing. 

 Blue whales. Fin whales. 



is relatively slow in the earlier stages. It is a characteristic feature of the development 

 of these whales that the form of the body is practically perfected at a stage when the 

 foetus is still very small. A 0-5 m. foetus, for instance, differs very little in appearance 

 and bodily proportions from the adult and so far as the internal structures are con- 

 cerned the organs are probably all laid down by the time the foetus has reached o-i m. 

 It is therefore natural to suppose that the actual linear rate of growth is extremely slow 

 while the foetus grows from zero to about o-i m. compared with its subsequent growth 

 up to the end of gestation, for from between o-i m. and 0-5 m. up to birth at 6-7 m. 

 development consists mainly in increase in size. 



It appears that the rate of growth in Fin whales is somewhat less than in Blue whales 

 and it is probable that in Sei whales it is slovi^er than in Fin whales. 



The dotted curves in Figs. 145 and 146 are reproductions of the mean curve of growth 



