450 CHARLES H. SWIFT 



In embryos having 33 pairs of segments (about 3 days of 

 incubation) the primordial germ-cells are distributed in the radix 

 mesenterii, the coelomic epithelium and the mesenchyme beneath, 

 on both sides of the coelomic angle. 



Recently von Berenberg-Gossler ('14), working with duck 

 embryos, has confirmed the findings of Swift ('14) in all the 

 essential points. The primordial germ-cells in this form, how- 

 ever, do not leave the blood vessels of the splanchnopleure until 

 the embryo has from 24 to 32 pairs of somites. 



LATER HISTORY OF THE PRIMORDIAL GERM-CELLS 

 1. Primordial germ-cells and the indifferent gonad 



As an introduction to this section dealing with the indifferent 

 sexual primordium, I must say that my findings, relative to this 

 period of development coincide in all essential details with those 

 of Firket ('14). For this reason, and because the reader may 

 refer to his excellent monograph, I shall pass over this period 

 of organogenesis rapidly. 



On opening the body of a 3f day (90 hours) chick, and on re- 

 moving the abdominal viscera so as to expose the posterior wall, 

 the Wolffian bodies are seen. They have a pink color and have a 

 length of about 3 mm.; their long axes correspond to the longi- 

 tudinal axis of the animal. 



On the medial surface of the rounded ventral face of each 

 Wolffian body — the surface towards the mesentery — there is 

 another elongated, narrow, whitish body, which has the appear- 

 ance of a ridge. This is the young developing gonad. The 

 whitish color of the area is due in part to the thickened bloodless 

 coelomic epithelium, the so-called germinal epithelium. The 

 gonads are about 1.5 mm. in length and their anterior extremi- 

 ties begin some distance behind the cephalad extremities of the 

 Wolffian bodies. Even at this early stage the hand lens, or even 

 the unaided eye, will show that the left gonad is more massive 

 than the right. 



