ORIGIN OF DEFINITIVE SEX-CELLS 453 



in the cat as coming from the epithehum of Bowman's capsule; 

 neither is this in accord with Allen's ('05, '06) findings in reptiles 

 fChrysemis) who described them as derived through invagi- 

 nations of the germinal epithelium. The opinion of Firket 

 ('14) that they are merely condensations of mesenchyme is, in 

 my opinion, correct. The cells of which they are composed 

 differ in no way from those of the general mesenchyme. They 

 merge without any change into the other portion of the ''organ 

 of Mihalkowics" under the epithelium and this in turn merges 

 with the general embryonic mesenchyme. The cells of these 

 cords are not epithelial in character and do not resemble the cells 

 of the germinal epithelium nor those of Bowman's capsule. The 

 primordial germ-cells in them can be explained by migration. 



The third tissue in the gonad is made up of primordial germ- 

 cells. They are present between the cells of the germinal epi- 

 thelium, on both sides of the coelomic angle, in the mesenchyme 

 tissue under the epithelium and in the cords of urogenital union 

 (fig. 1). They are not all confined to the gonad for some are 

 present in the germinal epithelium lining the base of the radix 

 mesenterii, while others are present in the mesenchyme of the 

 same structure. Most of them are found in the medial portion 

 of the gonad near the coelomic angle, although an occasional 

 one will be found in the germinal epithelium of the gonad far 

 removed from this spot. Their appearance at this age is the 

 same as that of those described by Swift ('14) in younger embryos. 

 The factors which distinguish them are size of cell, clearness 

 of cytoplasm, size of nucleus, persistent droplets of vitellus and 

 the large attraction-sphere (figs. 1 and 4). They are not numer- 

 ous — from one to ten in a single section — nor are they dividing 

 to any extent. One interesting point which will be taken up 

 at length in the next stage to be described is the fact that there 

 are more of them in the left gonad than in the right. 



In connection with the next stage, that of approximately 4 

 days, seven embryos were studied. The gonad has about the 

 same length but projects more into the coelom. This increase 

 in size is due in part to the increased thickness of the germinal 

 epithelium, which is now composed of two or three layers of cells. 



