564 SCIENCE PROGRESS 



whether these arise in the uterus, placenta, or foetus, must 

 be determined by experiment. That the impulses cannot be 

 nervous in character is clearly demonstrated by the experiments 

 of Eckhard and Ribbert, and especially by those of Goltz and 

 Ewald on the effects of extirpation of the spinal cord. Since 

 pregnancy occasions hypertrophy of the mammary gland, and 

 parturition is followed by the onset of lactation, in the total 

 absence of any possible nervous connection between the pelvic 

 organs and these glands, it is evident that the correlated growth 

 of the mammary glands must be determined by chemical sub- 

 stances arising in the pelvic organs and carried to the glands 

 by the blood-stream. Knauer has shown that, whereas extir- 

 pation of both ovaries puts an end to the periodical changes in 

 the uterus, which are responsible for the phenomena of " heat," 

 both ovaries can be transplanted, thus dividing all their nervous 

 connections, without abolishing the phenomena. In this case 

 therefore the connecting link must be chemical rather than 

 nervous. 



In the case of the mammary glands we have to determine 

 in the first place why the secretion of milk appears only at 

 the end of pregnancy, and in the second place the origin of 

 the stimulus which during pregnancy is responsible for the 

 hypertrophy of the gland. 



With regard to the first point, Hildebrand has suggested 

 that during pregnancy some substance circulating in the blood 

 exercises an inhibitory influence on the dissimilatory changes 

 in the gland cells, which he regards as autolytic in character. 

 Although it is extremely improbable that the chemical changes, 

 which characterise activity, are identical with the autolytic 

 changes occurring immediately after the death of the gland cells, 

 the conception of a substance causing growth by acting as an 

 inhibitory agent or, in Hering's nomenclature, having an assimi- 

 latory effect, is extremely valuable. According to this notion, 

 so long as the inhibitory substance is circulating in the blood- 

 stream, so long must growth of the mammary tissue proceed. 

 With the removal of the source of the inhibitory hormone, 

 which takes place at parturition, the gland tissue, built up to a 

 high level of function, will undergo autonomous dissimilation, 

 i.e. will enter into a state of prolonged activity. Miss Lane- 

 Claypon and I have found that artificial cessation of pregnancy 

 in the rabbit at any time during the first fourteen days — that is 



