190G.] Growth and Activity of the Mammary Glands. 521 



however, is required on this point, as also on the question whether the 

 substance is specific to the animal, or whether the injection of extracts of the 

 foetus of one animal would produce a growth of the mammary glands in 

 another species. One experiment, in which we fed a kitten for three weeks 

 on the foetuses of rabbits, was negative in its results. This might, however, 

 have been due to the failure of the intestine to absorb the hormone without 

 destruction, or to the failure of the immature glands to react to the minute 

 stimulus which they received. So far as we know, secretin is not absorbed 

 into the circulation when introduced into the stomach or intestine, and 

 colossal doses of adrenalin have to be given by the mouth in order to produce 

 any systemic effects. 



The effect of the injections of foetal extracts on muciparous rabbits 

 deserves some further mention. The multiparous 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 24 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 24. While it was circulating it should cause 

 building up of the secreting cells. Directly, however, it ceased to circulate, 

 the cells would enter into dissimilative 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 object of our experiment. 



Conclusions. 



So far as our experiments go, they show that the growth of the mammary 

 glands during pregnancy is due to the action of a specific chemical stimulus 

 produced in the fertilised ovum. The amount of this substance increases 

 with the growth of the foetus, and is therefore largest during the latter half 

 of pregnancy. Lactation is due to the removal of this substance, which must 

 therefore be regarded as exerting an inhibitory influence on the gland cells, 

 hindering their secretory activity and furthering their growtk It is probable 

 that the specific substance is diffusible, and will withstand the boiling 

 temperature. 



