ORIGIN OF DEFINITIVE SEX-CELLS 459 



origin, and that the primordial germ-cells found in the cords are 

 there because they happened to be .present in the localized 

 region of proliferation which gave rise to them (fig. 5). 



At this time, that is to say, during the 6th and 7th days of 

 development and in fact for some days to come, there are three 

 tissues in the mass beneath the germinal epithelium. The old 

 mesenchymal tissue between the cords, the cords derived from 

 the germinal epithelium, whose cells are like those of the parent 

 tissue, and the primordial germ-cells (fig. 5). The latter have 

 entered the stroma of the gonad in various ways, that is, some 

 of them have been in it from the beginning, others entered it 

 from the germinal epithelium by individual migration, while 

 still others were brought in by the cords of first proliferation. 



For the evolution and degeneration of the cords of first pro- 

 liferation in the female chick the reader is referred to an excellent 

 account written by Firket ('14). 



When the chick embryo has reached the 156th hour of develop- 

 ment (6| days) , the formation of cords of first proliferation ceases 

 rather abruptly and it is about this time that the sex of the 

 individual can be definitely determined. 



In the determination of sex there are three criteria on which 

 reliance can be placed. These are — the relative size of the two 

 gonads, the germinal epithelium, and the number of primordial 

 germ-cells in the germinal epithelium. 



In all the embryonic stages which I have studied the right 

 gonad is smaller than the left, but towards the end of the sexual 

 cord formation this disparity in size becomes marked in the 

 female. The great increase in size of the left gonad in the female 

 is due to several factors. In the first place the cords of first 

 proliferation in this gonad grow rapidly and there is also an 

 increase in the thickness of its germinal epithelium. 



The germinal epithelium of the left gonad of the female pos- 

 sesses several more layers of cells than does that of the male. 

 In the male gonads and in the right female gonads the epithelium 

 remains relatively thin. This increase in thickness of the germ- 

 inal epithelium takes place synchronously with the formation 

 of the cords of first proliferation. 



