THE PHYSIOLOGY OF LACTATION 39 



glands and at the same time prevents milk secretion. Lane-Clay- 

 pon and Starling are of the opinion that a substance passes from 

 the fetus into the maternal circulation. This substance stimu- 

 lates growth of the mammary glands during pregnancy and at the 

 same time inhibits milk secretion, although mammals sometimes 

 lactate up to the next parturition. With expulsion of the fetus 

 the stimulus disappears, growth of the gland ceases, there is no 

 further inhibition of milk secretion, and flow commences. These 

 authors produced enlargement of the mammary glands in a virgin 

 rabbit by injecting extracts of rabbit fetus, but milk secretion 

 was inhibited. The case of the Bohemian pygopagous twins is 

 cited in support of this theory. One of the pair became pregnant, 

 and during this period the mammary gland in both enlarged. 

 Ribbert transplanted a mammary gland of a guinea-pig into the 

 skin back of the ear of another female guinea-pig. Five months 

 later living young were born and milk was promptly secreted from 

 the transplanted gland. Gaines injected defibrinated blood into 

 a lactating goat — 1, from a pregnant goat near delivery; 2, from a 

 kid of the same goat two days after birth; 3, from the same goat six 

 days after delivery. In each case an inhibitory action was ob- 

 served. Injection of a cow's placenta into a lactating goat also 

 produced inhibition. Transfusion from a fresh, heavy milking 

 goat to one giving a low yield seemed to cause a decrease in the 

 milk of the injected goat. The author concluded from these trans- 

 fusion experiments that the hormone is present in the blood in 

 small quantity and that the gland of the pregnant female is more 

 susceptible to its influence than the gland of the normal female; 

 that lactation is not due to a hormone, because transfusion of blood 

 from an actively lactating goat to a feebly lactating one does not 

 increase secretion in the latter. Furthermore, one lactating gland 

 may cease lactating before the other. 



On the other hand, Lambroso and Bolaffio, working with para- 

 biotic rats, found that pregnancy of one of the pair did not affect 

 the mammary glands of the other. This might argue against the 

 presence of a hormone, but the authors showed that food does 

 not pass from one of the pair to the other, and consequently hor- 

 mones are probably not carried from the circulation of one to the 

 other. 



Although there are some dissenting voices, the evidence of the 

 presence of hormones in the blood is convincing, but whether 

 the placenta or the embryo is the source of the hormo;ne is a ques- 

 tion that cannot be answered. A hormone originating in the em- 

 bryo would have to pass through the placenta to reach the maternal 

 circulation, and even this would leave us in doubt as to the true 

 origin of the hormone. 



