262 INTERNAL SECRETIONS 



foetus. We must assume that an influence is exerted by the 

 placenta and the foetus by means of hormones. This influence 

 might be either a direct or an indirect one. In the first instance 

 substances from the placenta would be carried by the blood 

 to the mammary glands, and so act on them. In the second, 

 which is very probable, these substances would act also on 

 the ovary; that such an action really takes place is rendered 

 likely by the fact that the corpus luteum of pregnancy per- 

 sists longer than the corpus luteum of menstruation or the 

 periodic corpus luteum. The above-mentioned observations of 

 L. Loeb on the relation existing between an experimentally 

 produced placenta and the ovarian changes may be again 

 referred to in this connection. One may assume that the 

 influence the placenta has on the mammary glands may here 

 be indirect, the development of the mammary gland being 

 caused by an increased formation of endocrine cells in the 

 ovary, depending upon the placenta. Weighty evidence for 

 such an assumption is given by the above-mentioned experi- 

 ments of Steinach and Holzknecht on guinea pigs treated with 

 X-rays, where growth of the mammary glands and milk 

 secretion were observed without a placenta having been formed. 

 A further proof is given by the numerous experiments of 

 Steinach, Athias, Sand, Moore, Lipschiitz and his co-workers, 

 who observed milk secretion in male animals after implan- 

 tation of ovaries. But, on the other hand, it seems clear 

 that besides the influences exerted on the mammary gland 

 by the ovary, there must be still other factors at work, since 

 we know that the growth of the mammary glands does not 

 cease if castration is performed in the second half of pregnancy. 

 It may be that the changes produced during pregnancy in 

 the ovary are followed by changes in other ductless glands, 

 and there are many indications that this actually happens, 

 All these manifold factors must be taken into consideration 

 in dealing with the question as to the factors responsible for the 

 hypertrophy of the mammary glands in pregnancy and for 

 milk secretion after birth. 



