REPRODUCTION AND SEX 491 



THE ROLE OF INTERNAL SECRETIONS.— It has long been 

 recognised that the reproductive organs exert a pervasive influence 

 on the body, as is conspicuously seen in the changes that occur at 

 adolescence and in pregnancy even in remote parts of the body. 

 The fact is expressed in Helmont's aphorism: "Propter solum uterum 

 mulier est, quod est", which Chereau changed into "Propter solum 

 ovarium mulier est, quod est". 



The view of Pfliiger that the gonads exert an influence through 

 the nerves associated with them has given place to the view, origin- 

 ating with Brown-Sequard, that the influence passes into the body 

 by the medium of the internal secretions of the gonads. To Starling 

 we owe the convenient term "hormones" for the specific stimulating 

 substances in these internal secretions. 



It may be recalled that a male organ or testis in a higher animal 

 consists (i) of sperm-making cells arranged in tubules, (2) of inter- 

 stitial cells of various types, and (3) of a connective-tissue outer 

 envelope. Similarly the female organ or ovary consists: (i) of ova 

 disposed in groups or follicles; (2) of interstitial cells of various 

 kinds (the stroma of the ovary, the follicle cells, and the corpus 

 luteum), and (3) of a connective-tissue outer envelope. 



It has been shown by many investigators that the interstitial 

 cells of the MammaUan testis possess a relative independence of the 

 germinal portion. They may be well developed at a time when the 

 germinal part is still embryonic; they may occur at some distance 

 from the seminiferous tubules; they may be normal in old testes 

 from which the sex-elements have disappeared, or in diseased testes 

 in which only the seminiferous part is affected. Three functions have 

 been assigned to the secretion of these remarkable glandular cells, 

 that it is nutritive for the testis, that it acts as a formative stimulus 

 for the secondary sex-characters, and that it affects genital excite- 

 ment. 



It has been shown that masculine characters (e.g. in the horse and 

 in man) may develop although the sperm-making part of the testis 

 is degenerate, provided the interstitial part is well developed. 

 It follows that the stimulating internal secretion, without which the 

 masculine characters do not develop, is produced by the interstitial 

 tissue. It has been shown (e.g. in mole, marmot, man) that the 

 interstitial tissue waxes and wanes, and that the recurrence of 

 "heat" in animals is preceded by activity of the interstitial tissue 

 before sperm-making activity sets in. Similarly in the female the 

 internal secretions that pass from the ovary have their origin not 

 in the germinal but in the interstitial part of the organ. 



Let us consider the case of fowls. For three or four weeks after 

 hatching, chickens do not show any external marks of sex. In size 

 and colour of comb, in plumage and limbs, pullets and cockerels are 

 alike, and it is not till towards the thirtieth dav that the external 



