THEORY OF SEX-DIMORPHISM 81 



the ovary, the follicle cells, and the corpus 

 luteum), all within the sheath of connective 

 tissue. 



It has been shown by many investigators 

 that the interstitial cells of the mammalian 

 testis possess a relative independence of the 

 germinal portion. They may be well devel- 

 oped 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 

 excitement. 



It has also 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, so long as the 

 interstitial tissue remains well developed. It 

 follows that the stimulating internal secretion, 

 without which the masculine characters do 

 not develop, is produced by this interstitial 

 tissue. In mole, marmot, man and other 

 mammals, the interstitial tissue waxes and 

 wanes, and the recurrence of " heat " in 

 animals is preceded by activity of the inter- 

 stitial 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 inter- 

 stitial part of the organ. 



