REGULATORS 609 



Removal of the ovaries late in pregnancy produces no 

 disturbances. 



(2) The Mammary Gland. — When the ovaries are removed in 

 early life the mammary gland does not grow. The growth of 

 the gland in pregnancy seems to be associated with the develop- 

 ment of the corpus luteum. It has been found that, in the 

 rabbit, the injection of extracts of the foetus leads to a rapid 

 growth and to a functional activity of the gland, and it has been 

 deduced that a secretion from the foetus is the specific stimulus 

 to the development of the gland and to milk formation. But 

 milk secretion occurs after the expulsion of the foetus, and it is 

 more probable that the retention in the mother of the material 

 which was formerly passed to the foetus, and which is virtually 

 foetal material, is the stimulus to milk secretion, rather than 

 that this is caused by a reabsorption from the foetus. That an 

 internal secretion from the foetus does not play an essential part 

 is shown by the fact (i.) that milk secretion may occur in 

 the young of both sexes; and (ii.) that it may be caused 

 by stimulation through the nervous system, since it has been 

 induced, even in the virgin animal, by continued stimulation 

 of the nipple by sucking ; (iii.) that milk may be produced in 

 bitches some weeks after oestrus without impregnation. In 

 such bitches development of the corpus luteum and of the 

 uterus and mammary glands occurs and retrogressive changes do 

 not appear till after thirty days. This may explain the secre- 

 tion of milk which in these cases seems to follow atrophy of 

 the corpus luteum, while increase of the mammarv gland is 

 associated with its growth. 



10. Inter-renal Tissue. 

 In mammals, this is chiefly massed as the cortex supra- 

 renalis, but separate masses occur along the course of the aorta, 

 in the epididymis testis and near the ovaries. In some animals, 

 e.g. the rat, they are more abundant than in others. In fishes 

 the inter-renals lie quite separately from the chromaffin tissue. 



(1) Development. — The tissue is developed from ingrowths 

 of the mesothelium of the genital ridge. 



(2) Structure. — The cells are large, and the protoplasm con- 

 tains an abundance of lipoids, with a high proportion of 

 cholesterol. They closely resemble the interstitial cells of the 



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