374 EMBRYOLOGY. 



off ; however, the similarity with the conditions found in the Chick 

 and the non-amniotic Vertebrates is not to be denied, and has indeed 

 been emphasised by NAGEL. 



(e) The Germinal Epithelium. 



In Vertebrates, at the time when the Miillerian duct is established,, 

 the first traces of the sexual glands are also to be recognised. The 

 parent-tissue of these is likewise the epithelium of the body-cavity. 

 This acquires for example in the Chick, which is to serve as the 

 foundation for our description a different appearance in the various 

 regions of the body-cavity (fig. 210). In most places the epithelia be- 

 come extraordinarily flattened and assume the condition of the perma- 

 nent " endothelium." Also on the mesonephros, which projects into the 

 body-cavity as a thick, vascular fold, the epithelium is for the most 

 part greatly flattened, but retains its original condition (1) on its 

 lateral surface along a tract (a') from which, as we have previously 

 seen, the Miillerian duct is formed, and (2) along a tract (a) which 

 stretches from in front backward along the median side of the 

 mesonephros ; the signification of the latter has been correctly 

 estimated by BORNHAUPT and by WALDEYER, who have characterised 

 it as germinal epithelium. From it are derived the germ-cells : in the 

 female the primitive ova, in the male the primitive seminal cells. It is 

 only in the very earliest stages that it is impossible to distinguish 

 whether the germinal epithelium will be developed into testis or ovary* 

 Differences soon appear, which allow a positive determination. We 

 shall take up first the development of the ovary, then that of the 

 testis. 



(/) The Ovary. 



The development of the ovary is tolerably well known both in the 

 lower and the higher Vertebrates, except for a few controversial points. 

 I can therefore limit myself simply to the presentation of the results 

 which have been acquired in the case of the Chick and Mammals. 



At about the fifth day of incubation the germinal epithelium in 

 the Chick increases a good deal in thickness, becoming two to three 

 layers of cells deep. Certain elements in this thickening are promi- 

 nent ; they are distinguishable (fig. 210 C and o) by their richness 

 in protoplasm and by their large round nuclei. Because they stand 

 in the closest relation to the development of eggs, they have been 

 designated as primitive eggs by WALDEYER, who was the first to 

 study them in detail. 



