DIFFERENTIATION OF THE SEXES. 401 



which insert themselves, to a certain extent, between the 

 endocoelar and the exoccelar, there forming the germ- 

 plate, cannot be referred either to the intestinal-fibrous 

 layer or to the skin-fibrous layer, but directly to the two 

 primary germ-layers; for there are important grounds for 

 supposing that even the first rudiment of the sexual plate is, 

 probably, hermaphroditic, and that this " sexual epithelium" 

 (visible, in Man and all other Vertebrates, between the exo- 

 ccelar and- the endoccelar) represents a primaeval and simple 

 hermaphrodite gland. (Cf. vol. i. p. 256, Figs. 52-56, e, h.) 

 The inner half of this, in contact with the intestinal-fibrous 

 .ayer, which is derived from the intestinal-glandular layer, 

 would be the rudiment of the ovary; its outer half, in 

 contact with the skin-fibrous layer, which originates from 

 the intestinal -glandular layer, would be the rudiment of the 

 testes. This is, of course, only conjectural. 



We ought, accordingly, to distinguish two different 

 sexual plates or germ-epithelia ; the female sexual plate, a 

 product of the intestinal layer, which gives rise to the 

 ovary-epithelium the mother cells of the ova (" ovary- 

 plate ") ; and the male sexual plate, lying externally over the 

 former, and which is a product of the skin-layer, from which 

 originates the testes-epithelium the mother cells of the 

 sperm-threads (" testes-plate ") ; but even the first recog- 

 nizable rudiments of the two sexual plates appear, indeed, 

 BO intimately associated in the human embryo and in those 

 of the higher Vertebrates, that hitherto they have been re- 

 garded as a single, undifferentiated, common rudiment of an 

 organ ; and it is still possible that the two kinds of sexuai 

 glands arise by secondary differentiation from a common 

 rudiment. 



