88 INTERNAL SECRETION 



tubes, as in cases of tubal pregnancy. The readiness with 

 which extra-uterine pregnancy may develop in any species 

 depends, in part at least, upon the readiness with which the 

 stroma of the host responds with the production of a decidua 

 favourable for the development of the embryo. 



It is probable, then, that the corpus luteum aids in the fixa- 

 tion of the embryo by favouring the growth of the deciduoma 

 in response to the stimulus of the ovum. But recent investi- 

 gations point strongly to the view that the yellow body has an 

 important function related to the mammary gland. 



It was suggested by Hildebrandt that during pregnancy an 

 impulse is exerted by the ovum on the mammary glands, which 

 stimulates them to growth. Lane-Claypon and Starling re- 

 ported that extracts of foetus stimulate the growth of mammary 

 gland. But virgin animals sometimes produce milk. Hence 

 it is clear that the source of the stimulus which normally 

 calls forth the development of the mammary gland must be 

 sought elsewhere than in the developing foetus. Frank 

 V. linger and O'Donoghue have definitely urged that this 

 source is to be found in the corpus luteum. The last-named 

 author, working upon Dasyurus, has found that the enlarge- 

 ment of the mamma begins quite apart from fertilization, 

 and is continued when there is no fertilized ovum present to 

 produce an internal secretion. He gives tables and curves to 

 show that as soon as the corpus luteum has begun to be formed, 

 the growth of the mammary gland commences, and this 

 growth is notably increased after the corpus luteum is fully 

 formed. As we have seen above (p. 86) O'Donoghue observed 

 that, if follicular rupture is not followed by formation of cor- 

 pora lutea, there is no growth of the mammary glands. The 

 conclusion can scarcely be avoided that the hormone causing 

 the growth of the mammary gland is produced in and secreted 

 by the corpora lutea. 



Reference has already been made to the " interstitial cells " 

 or the "interstitial gland" ("puberty gland" of Steinach). 

 The tissue is so often referred to in the literature, that it is 

 astonishing how little can be definitely stated about it. It 

 is not known certainly whether the tissue is epithelial or whether 

 it is of connective tissue origin. It is not even known whether 

 it is constantly present. In fact, out of more than one hundred 

 species so far examined, 50 per cent, are said to possess none. 





