ORIGIN OF DEFINITIVE SEX-CELLS 457 



2. Primordial germ-cells, germinal epithelium and medullary cords 

 of the embryonic ovary 



The next three embryonic stages studied, namely, 5|, 6 and 6^ 

 days, are of great interest, for in them the first true sexual cords, 

 or cords of first proliferation, make their appearance. In the 

 male they become the semeniferous tubules and in the female 

 form the medullary cords which later have much to do with the 

 constitution of the ovarian medulla. 



The gonad of the 5|-day chick has a stroma, which is composed 

 of onl}^ one kind of element, the mesenchyme tissue. The cells 

 of this tissue are more closely packed than in the general embry- 

 onic body, but their round or branching character can readily be 

 seen. There are many karyokinetic figures in this tissue, in 

 which are to be seen also a few primordial germ-cells. 



The general thickness of the germinal epithelium remains 

 about the same but in several places there are projections or buds 

 on its deep surface. An occasional finger-like projection of the 

 epithelium into the stroma is present. The bud is the beginning, 

 and the projection is the completion of the cord of first prolif- 

 eration. These buds and the completed cords are due to local 

 increase in activity of the germinal epithelium. It is true that 

 they may contain one to several primordial germ-cells, but I 

 have never seen any evidence that they played any role other 

 than a passive one in the local hyperplasia, which gives birth 

 to the sexual cord (fig. 5). 



The cords of first proliferation are seen at their best in the 

 next two stages — embryos of 6 and 6| days respectively. 



In these stages the evolution of the cords is to be seen — the 

 bud, the cord still attached to the germinal epithelium, and the 

 cord which has become separated from the latter by an ingrowth 

 of stroma (fig. 5). In this connection it may be said that the 

 growing cord does not remain attached to the epithelium for 

 any length of time. At first the basement membrane of the 

 epithelium is continuous around the lengthening cord but very 

 soon this continuity is lost (fig. 5). After separation from their 

 mother tissue they become autogenous and increase rapidly 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 18, NO. 3 



