444 CHARLES H. SWIFT 



denying that some could originate there, believed that others 

 might arise by a process of differentiation either from the cells 

 of the sexual cords, or from the mesenchyme cells which make 

 up the columns of cells connecting the Wolffian body with the 

 germinal epithelium in the young embryo. 



If the origin of these primordial germ-cells was in doubt their 

 fate was certainly a greater mystery. According to some — Wal- 

 deyer and Semon, for example — they developed directly into the 

 definitive ova, while Prenant would not hazard a guess as to 

 their issue. 



It remained for Hoffmann to prove that the primordial germ- 

 cells were a thing apart from the elements making up the germinal 

 epithelium and ovarian stroma; that they existed a long time 

 before the appearance of the germinal epithelium and gonad, and 

 that the definitive ova were in a direct line of descent from the 

 true primordial germ-cells. 



Hoffmann employed in his work twelve species of birds, includ- 

 ed in the orders Natatores and Grallatores. In three species, 

 Haematopus ostralegus. Sterna paradisea, and Gallinula chloro- 

 pus, there was sufficient evidence brought out to prove that the 

 primordial germ-cells did not originate in the modified coelomic 

 epithelium. In the three species mentioned above, he found at 

 the proper time, numbers of the primordial ova in the germinal 

 epithelium. But, in addition, he found cells — supposedly pri- 

 mordial ova, because of their resemblance to those found later 

 in the germinal epithelium — in embryos of 23 somites. An 

 embryo of 23 somites does not possess the so-called germinal 

 epithelium, the coelomic epithelium over the Wolffian body not 

 having been modified at this age, yet in these he found primordial 

 germ-cells far removed from the site of the future sex-gland, in 

 the splanchnic plate of mesoderm, in the region between splanch- 

 nic mesoderm and entoderm and in the entoderm itself. 



In another form, Numenius arcuatus, Hoffmann worked out 

 the later history of the primordial ova. He observed that the 

 primordial germ-cells at first remained quiescent in the thicken- 

 ing germinal epithelium. When the germinal epithelium sent 

 down into the underlying stroma of the gonad the first series of 



