Physiologists, however, are not equally in agreement concerning the source of the hormone which regulates lactation. Starling and Miss Lane-Claypon concluded from their experiments on rabbits that the hormone originated in the foetuses themselves within the pregnant uterus. In virgin rabbits it is difficult to find the milk glands at all. When found the nipple is minute and sections through it show the gland to consist of only a few ducts a few millimetres in length. Five days after impregnation the gland is about 2 cm. in diameter. Nine days after impregnation the glands have grown so much that the whole inner surface of the skin of the abdomen is covered with a thin layer of gland tissue. In six cases by injecting subcutaneously extracts of foetus tissue Starling and Lane-Claypon obtained a certain amount of growth of the milk glands. The hormone in the case of the pregnant rabbit is of course acting continuously for the whole period of pregnancy, while the artificial injection took place only once in twenty-four hours, and the amount of hormone it contained may have been absorbed in a very short time. The amount of growth obtained experimentally in five weeks was less than that occurring in pregnancy in nine days. Extracts of uterus, placenta, or ovary produced no growth, although the ovaries used were taken from rabbits in the middle of pregnancy. In one experiment ovaries from a pregnant rabbit were implanted into the peritoneum of a non-pregnant rabbit, but on post-mortem examination of the latter eleven days later the implanted ovaries were found to be necrosed and no proliferation of milk gland had taken place. The conclusions of Starling and Lane-Claypon were confirmed by Foa, [Footnote: _Archivo d. Fisiologia_, v., 1909.] and by Biedl and Königstein, [Footnote: _Zeitschrift f. exp. Path. und Therap_., 1910.] Foa states that extracts of foetuses of cows produced swelling of the mammae in a virgin rabbit.