566 SCIENCE PROGRESS 



than give these glands a series of small shoves in the same 

 direction. 



In spite of the inherent difficulties of the research, we suc- 

 ceeded in six cases in producing in virgin rabbits a growth of 

 the mammary glands, similar to that occurring during the early 

 stages of pregnancy, and consisting in the proliferation of the 

 epithelium lining the ducts, and the multiplication of these ducts 

 by branching into the surrounding tissues. In one of these 

 experiments, where our injections were carried out during five 

 weeks, and the rabbit received the fluid extract of as many as 

 1 60 foetuses, there was an actual formation of secreting acini 

 towards the periphery of the gland. In all these cases the 

 extract was derived from foetuses. In a number of other 

 experiments in which we injected extracts of uterus, placenta, 

 or ovaries, we obtained no growth whatsoever. We are there- 

 fore justified in concluding that, under normal circumstances, 

 the growth of the mammary gland during pregnancy is deter- 

 mined by a chemical substance, a hormone, which is produced 

 mainly in the growing embryo, and is carried through the 

 placenta by the blood-stream to the gland. The smallness of 

 the effect obtained in our experiments, in comparison with 

 the large amount of tissues employed, shows that the quantity 

 of this hormone present at any given time in the tissues 

 must be minimal, and that, in all probability, when injecting 

 extracts of foetus, we are simply injecting the small amount of 

 material which is diffused through the juices on its way to the 

 blood-vessels and into the maternal circulation. 



Our experiments throw no light on the seat of production 

 of the mammary hormone in the foetus, nor do we yet know 

 whether it may be split off by simple means from some pre- 

 cursor in the foetal tissues, and so obtained in larger quantities, 

 as is the case with pancreatic secretin. We have a certain 

 amount of evidence that in one respect the mammary hormone 

 resembles secretin or adrenalin, i.e. in the fact that it can be 

 boiled without being deprived of its properties. The other 

 questions as to the seat and nature of the specific substance, 

 as well as the influence of various reagents in splitting it 

 off from some possible precursor, must be left to future 

 investigations. 



These three examples may serve to show that it is possible 

 by chemical means to influence either the functional or the 



