ON SOMATIC SEX-CHARACTERS 103 



performed after the fourteenth day, milk appears 

 within two days after the operation. It is to be 

 concluded from this that the cause of secretion of 

 milk is the withdrawal of a stimulus proceeding from 

 ovary or uterus. But O'Donoghue believes that 

 milk is secreted in Dasyurus when no pregnancy has 

 occurred. Ancel and Bouin l have shown that the 

 growth of the mammary glands was produced in 

 rabbits by the artificial rupture of egg follicles 

 and consequent production of corpora lutea : the 

 growth of the glands continued up to the fourteenth 

 day, after which regression set in. This shows that 

 the development of the milk glands in rabbits is due 

 to the corpora lutea. On the other hand, Lane- 

 Claypon and Starling state that in rabbits the 

 corpora lutea diminish after the first half of preg- 

 nancy, while the growth of the milk glands is many 

 times greater during the second half than during 

 the first half of the period, and during the second 

 half the ovaries may be removed entirely without 

 interfering with the course of pregnancy or the 

 normal development of the milk glands. It is 

 evident, therefore, that in rabbits, whatever influence 

 the corpora lutea may have in the first half of preg- 

 nancy, they have none in the second half, and that 

 at this period the essential hormone proceeds from 

 the developing fcetus or foetal placenta. Again, if 

 it is the withdrawal of a hormone stimulus which 

 changes the milk gland from growth to secretion, it 

 cannot be the corpora lutea which are exclusively 

 concerned even in Dasyurus, for they persist during 

 lactation, while secretion begins shortly after par- 

 turition. 



1 C. R. Soc. de Biol, t. Ixvii., 1909. 



