33 



But in each case from the ninth injection onwards typical milk could be 

 obtained on squeezing the nipples. 



During the last 17 days of this experiment two virgin rabbits were 

 obtained and were used for trying the effects once more of placenta, 

 uterine mucous membrane, and ovaries. The first received the fluid 

 extract unboiled of 123 placentae mixed with the fluid extract of the 

 mucous membrane of 14 uteri. The other received 13 injections com- 

 posed of the fluid extract of 26 pregnant ovaries. In both cases at the 

 end of the experiment the uteri of the animals which had received the 

 injections were found to be somewhat enlarged and congested, but in 

 neither case was there any trace of growth of the mammary glands. 



A consideration of all these results brings us to the conclusion that the 

 specific stimulus which determines the growth of the mammary glands in 

 pregnancy is produced in the product of conception, i.e., the fertilised 

 ovum or foetus ; that it is contained in all parts of the foetus ; that in all 

 probability it withstands boiling ; that it passes through a Berkefeld filter, 

 and is not retained to any appreciable extent by the Kieselgur in 

 Buchner's method for the extraction of cell juices. This mammary 

 hormone has apparently a two-fold effect, according as the animal experi- 

 mented on is multiparous or primiparous. If the animal be a virgin, the 

 effect of its injection is to produce hyperplasia of the gland, beginning, as 

 in normal pregnancy, with proliferation of ducts and later leading to for- 

 mation of secreting alveoli. In the multiparous animal, where there is 

 already a considerable amount of partly involuted mammary tissue, the most 

 striking result of the injection of this hormone is the production of 

 secretion of milk. 



The question arises whether these two results are to be ascribed to 

 two substances or whether they represent phases in the action of one 

 substance. A little consideration will show us that in all probability the 

 latter is the true explanation. It has already been suggested by Hilde- 

 brandt that the specific hormone for mammary growth is a substance 

 which inhibits autolytic processes in the gland. It is highly improbable 

 that the process of secretion can be ascribed to autolysis. But the idea 

 of an inhibitory substance as a stimulant to growth is a valuable one and 

 consonant with our ideas of the inhibitory process in general. The 

 stationary condition of any given cell is not the expression of inactivity 

 but is the result of the equilibrium between two sets of processes, one set 

 anabolic or assimilative, causing a building up of the cell, the other set 

 katabolic or dissimilative, causing a breaking down of the cell. During 

 activity of a muscle or gland, dissimilative processes predominate, causing 

 a loss of material. During the subsequent period of rest, there is a swing 

 back in the direction of increased assimilation, resulting in the making 

 good of the loss during activity. The continued application to any tissue 

 of a stimulus to dissimilation will finally result in its complete destruction 

 and death. On the other hand, the continued application of an assimila- 

 tory stimulus, if such an expression may be allowed, results in a piling up 

 of the sources of energy of the cell, with actual growth and probably 



