582 THE PHYSIOLOGY OF REPRODUCTION 



ferred to the maternal circulation at the time of birth. These 

 theories are based mainly on clinical evidence of a somewhat 

 questionable value. 



Miss Lane-Clay pon and Starling have shown that after multi- 

 parous rabbits are injected with foetal extract milk is secreted by 

 the glands. This result is explained as follows : " The multi- 

 parous rabbit differs from a virgin rabbit in possessing ready- 

 formed alveoli, i.e. secretory structures. On the theory which we 

 have adopted, the circulation of the mammary hormone should 

 diminish any secretion in these alveoli, and should cause growth. 

 In all our experiments at least twenty-four hours elapsed between 

 each two injections. It is probable that the hormone was rapidly 

 absorbed from the injection, and was therefore present in the 

 blood of the animal only for a certain fraction, say a few hours, 

 out of the twenty-four. While it was circulating it should 

 cause building up of the secreting cells. Directly, however, it 

 ceased to circulate, the cells would enter into dissimulative 

 activity resulting in secretion. By our injections, therefore, we 

 are not able to imitate the continuous stimulus of pregnancy. 

 We are rather producing each day a pregnancy of a few hours 

 followed by a parturition. These factors should therefore 

 result in the production of milk in any animals possessing the 

 structures (i.e. the alveoli) which are capable of secreting 

 milk, and would therefore account for the secretion of milk 

 observed by us in all the cases where multiparous rabbits were 

 the objects of our experiment." 



It has been shown that in the foetus itself there is an 

 increased growth of the mammary glands during the last part of 

 pregnancy, while it is well known that a secretion is often formed 

 in the glands of the newly born. Halban has explained this 

 secretion as the result of removal of the inhibitory influence 

 that is to say, it is due to the same circumstance as the secretion 

 in the mother. Miss Lane-Claypon and Starling point out that 

 the complete change which occurs in the environment of the 

 newly born animal must induce equally profound changes in the 

 metabolism, and there is consequently no difficulty about the 

 conclusion that the formation of the mammary hormone ceases 

 with the commencement of extra-uterine life. 



The general conclusions reached by these authors may be 



