34 



multiplication of the cells. The final result of this continued building 

 up must be an increase in the functional capacity of the tissue, so 

 that it will be ready to break down i.e., enter into activity, directly 

 there is any diminution of the assimilatory stimulus. This is what 

 happens in the case of the mammary glands. During the whole of 

 pregnancy the mammary gland is receiving a continual inhibitory or 

 assimilative stimulus from a substance produced in the fertilised ovum 

 or foetus which, diffusing through the placental villi, circulates in the 

 maternal blood-vessels. The result of this negative stimulus is a building 

 up of a gland, proliferation of its cells, and formation of new secretory 

 alveoli. As soon as parturition occurs, the source of the inhibitory 

 stimulus is removed. As the substance which has already obtained 

 entrance in the maternal organism is used up, the cells built up to a high 

 state of activity begin to break down ; the dissimilation which is the con- 

 comitant of activity in the mammary cells resulting in the production of 

 milk. At any time during the course of pregnancy in an animal we can 

 remove the source of the inhibitory hormone by excision of the foetuses or 

 pregnant uterus. We have found that if extirpation of the pregnant 

 uterus be carried out during the first half of pregnancy i.e., at a time 

 when the mammary gland consists almost exclusively of proliferating 

 duct tissue the sole result is to bring about atrophy of the tissue already 

 formed. After about the fifteenth day in the rabbit, extirpation of the 

 pregnant uterus causes the appearance of milk in the gland alveoli, and 

 this milk can be obtained within two days of the operation on squeezing 

 the nipples. In the same way we may explain the secretion of milk 

 which is observed on injecting multiparous rabbits with extract of fcetus. 

 The daily injection of a small amount of extract of fcetus can be but a 

 poor imitation of the continual leakage of the specific hormone into the 

 maternal blood-vessels, which occurs during pregnancy. We must, in 

 fact, imagine that, during the first few hours after the injection, we are 

 really imitating the condition in pregnancy. The specific hormone will, 

 however, be probably absorbed and used up long before the termination 

 of the 24 hours which elapse between each injection. With the disap- 

 pearance of the hormone the cells already built up beyond their normal 

 point will tend to break down. There are, in fact, every day a temporary 

 pregnancy and a parturition, and in all cases where there is a glandular 

 epithelium present at the beginning of the experiment i.e., in all rabbits 

 which have been previously pregnant we shall have a tendency to 

 formation of milk in the gland during a certain number of hours in every 

 day of the experiment. 



We must conclude, therefore, from these experiments that the growth 

 of the mammary glands during pregnancy is due to the assimilatory or 

 inhibitory effects of a specific hormone produced in the body of the foetus 

 and carried thence through the placenta by the fcetal and maternal circu- 

 lations. The removal of this inhibitory stimulus at the end of pregnancy 

 determines the spontaneous break-down of the built-up tissues i.e., 

 activity which in those cells is expressed by the formation of milk. 



