398 DISCOVERY REPORTS 



Maturity is reached in Blue females at about 237 m. and in Fin females at about 20-0 m. 

 and it will be noticed that just before these lengths are reached the still immature 

 uterus undergoes an acceleration in its growth, and by the time maturity is reached 

 it has become conspicuously bigger, measuring not less than 13 cm. in Blue whales, 

 or (with one exception) less than 11 cm. in Fin whales. Fluctuations of course take 

 place subsequently in the size of the uterus, but once the functional enlargement has 

 taken place it never returns to its original size. 



30 



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LENGTH OF WHALE IN METRES 



Fig. 132. Fin whales. Size of uterus in whales of diflferent classes and different sizes. 

 • Immature. • Resting, o Recently ovulated. © With very small foetus. x Lactating. 



In " resting" whales (i.e. those mature females which are neither ovulating, pregnant 

 nor lactating) the average size of the uterus is about 17 cm., but, as will be seen in 

 Figs. 131 and 132, it may range from less than 10 cm. to over 25 cm. A new increase 

 in size, however, takes place at ovulation for it will be seen that the size of the uterus is 

 clearly above the average in those whales in which a corpus luteum of ovulation or an 

 early foetus not yet big enough to necessitate an increased size, has been found. 



In pregnancy the uterus grows to an enormous size, for the foetus reaches a length 

 of 6 or 7 m. before it is born, but after parturition involution takes place with surprising 

 rapidity. In almost all the lactating whales examined involution has been complete. 



