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and Jolly, who conclude that the ovary is an organ providing an internal 

 secretion, which is elaborated by the follicular epithelial cells or by the 

 interstitial cells of the stroma. This secretion circulating in the blood 

 induces menstruation and heat. In animals which have been deprived 

 of their ovaries and in which the phenomena of heat are therefore absent, 

 these phenomena can be reinduced by the injection of ovarian extracts. 

 After ovulation the corpus luteum is formed. This organ provides a 

 further secretion, the function of which is essential for the changes taking 

 place during the attachment and development of the embryo in the first 

 stages of pregnancy. 



A still more striking example of growth in response to chemical 

 stimulation from distant organs is afforded by the mammary glands. As 

 is well known, at birth these glands are limited to a few ducts in the 

 immediate neighbourhood of the nipple, continuous with those of the 

 nipple and equal in extent in both sexes. At puberty in the human 

 female there is growth of the breasts associated with some gland-growth, 

 the main increase in size, however, being due to fat. With the occur- 

 rence of pregnancy a true hypertrophy of the gland begins at once and 

 continues steadily up to birth. In the rabbit, in which we have studied 

 the changes in the gland, it is extremely difficult to find in the virgin even 

 a trace of mammary gland. The nipple is small and undeveloped and, 

 on making serial sections through the nipple, the gland is found to be 

 confined to a few ducts not extending more than a few millimetres out- 

 side the nipple. No trace of secreting alveoli is to be observed. With 

 the occurrence of pregnancy a rapid growth of the gland appears to begin 

 at once. Five days after impregnation, when it is still impossible to find 

 the impregnated ovum with the naked eye in the enlarged uterus, the 

 mammary glands are marked out as small pink patches about two centi- 

 metres in diameter just under each nipple. On microscopic section the 

 gland is found to be made up chiefly of ducts, which, however, are under- 

 going rapid proliferation. The cells lining the ducts are about three deep 

 and present numerous mitotic figures. At about the fourteenth day the 

 whole of the front of the abdomen is covered with a thin layer of mam- 

 mary tissue. Branching ducts with proliferating epithelium are still the 

 predominant feature on section, but here and there, especially towards 

 the margins of the gland, small secreting alveoli lined with a single layer 

 of epithelium are to be seen. After this time the gland grows with ever- 

 increasing rapidity, so that at birth, at the thirtieth day after impregnation, 

 the mammary glands form a layer about half a centimetre thick over the 

 whole of the abdomen. In the virgin rabbit it is impossible to obtain by 

 expression any fluid from the nipples, but from the fifth to about the 

 twenty-fifth day pinching the nipples results in the expression of a clear, 

 colourless fluid. From the twenty-fifth day onwards this fluid becomes 

 opalescent and during the second and third days immediately preceding 

 birth the fluid obtained is typical milk. The appearance of milk is earlier 

 in multiparous rabbits, and in animals where pregnancies succeed each 

 other rapidly it may be possible to express milk throughout the whole 



