CHEMICAL CORRELATION 565 



before any secretory alveoli have been formed — gives rise merely 

 to retrogressive changes in the gland. If, however, pregnancy 

 be brought to an end at any later period, activity is set up in the 

 secretory alveoli, and a secretion of milk is the result. That 

 this secretion is due to the removal of a stimulus and not to the 

 production of a new stimulating substance in any of the invo- 

 luting sexual organs is shown by the fact, well known to 

 clinicians, that lactation ensues even after complete removal of 

 the pregnant uterus with its appendages. 



With regard to the second question, viz. the origin of the 

 inhibitory hormone, the fact that double ovariotomy during 

 pregnancy does not interfere with the hypertrophy of the mam- 

 mary glands excludes the ovaries as the direct source of the 

 stimulus. A careful study of the clinical evidence has led 

 Halban to the belief that the source of the hormone is to be 

 found in the chorionic villi and the placenta. His evidence is 

 not, however, absolutely convincing, and we therefore sought to 

 determine the question by the injection of extracts of foetuses, 

 of ovaries, of placentae, and of uterine mucous membrane into 

 virgin rabbits, in the hopes of producing, by one of these means, 

 a hypertrophy of the mammary glands, similar to that which 

 would be produced by the occurrence of pregnancy. It was 

 evident to us, before we began our researches, that it would be 

 difficult, if not impossible, to present any stimulus to the mam- 

 mary glands which would be as effective as the normal one. 

 For wherever the mammary hormone is manufactured, the 

 manufacture must be assumed to proceed continuously. There 

 is therefore a constant leakage of the active substance into the 

 blood, and it is probable that the amount of this substance pro- 

 duced increases with the duration of pregnancy. At no time 

 will the mammary gland be set free from the influence of this 

 specific stimulus. On the other hand, however we might 

 prepare our extracts of the tissues, we could not expect to get 

 more than the amount residual in the tissue and caught, so to 

 speak, in its progress through the placenta into the maternal 

 blood-vessels. This amount we might inject into our rabbits, 

 but it would probably be taken up and absorbed into the circu- 

 lation long before we were ready for our next injection, so that, 

 whereas under normal circumstances the mammary glands 

 during pregnancy are being constantly stimulated to hyper- 

 plasia, we could not expect in our experiments to do more 



