Embryonic History of the Germ-Cells of the Loggerhead Turtle. 333 



dering cells" or " primordial germ-cells" in the lizard and the chick is 

 obviously difficult if not actually impossible on this basis. 



Rubaschkin (1908, 1909) describes an entodermal origin of primor- 

 dial germ-cells also in rabbit and guinea-pig embryos; and he traces a 

 migration route practically identical with that here described for 

 Caretta and that described for Chrysemys by Allen. In the cat embryo, 

 Winiwarter and Sainmont (1909) describe similar sex-cells in the 

 embryonic ovary, but they interpret these cells as hypertrophied forms 

 of the ordinary cells of the sex-cords. 



Nagel (1889) described extra-regional germ-cells also in the human 

 embryo, located near the pronephric duct. In 1911, Felix likewise 

 described them in human embryos. In 1912, Fuss published his results 

 of a study of 3 human embryos (of 2, 3, and 4 weeks) and 17 other 

 mammalian embryos, including rabbit embryos and pig embryos (of 7 

 to 14 mm.). In the human embryo the sex-cells are said to be capable 

 of amoeboid motion ; this movement is combined with a passive migra- 

 tion, the result of growth in the enveloping tissues. Fuss declares that 

 aside from the germinal path there are no germ-cells, and that the 

 so-called ''germinal epithelium" plays no large role in the origin of 

 germ-cells in mammals. The germ-cells are said to be present long 

 before the differentiation of the sexual gland takes place. They arise 

 from the entoderm, and after closure of the gut they migrate through 

 the mesentery to the genital gland in the human embryo about the 

 fourth week, in the rabbit embryo about the thirteenth day, and in the 

 pig embryo of about 14 mm. On entrance into the gland they may 

 undergo active mitotic proliferation. 



DISCUSSION AND CONCLUSIONS. 



It would seem that the evidence is now sufficient to amply support 

 the hypothesis of the extra-regional origin of the germ-cells first clearly 

 enunciated by Nussbaum (1880) in opposition to the teaching of 

 Waldeyer (1870), who derived the germ-cells by process of differ- 

 entiation from the peritoneal epithelium. Swift's most recent pub- 

 lications (1915, 1916) clear also the additional obscurity regarding 

 the germ-cells produced by Felix, who claimed that the primordial 

 germ-cells ("primary genital cells") in amniotes, including man, have 

 an extra-regional origin, but later disappear and become replaced by 

 "secondary genital cells," the definitive germ-cells, differentiation 

 products of the peritoneal epithelium. This is also the view of Dustin 

 regarding certain amphibia and Chrysemys, and of Gatenby for the 

 frog. Swift has shown for the chick that the sex-cords arise as 

 ingrowths of proliferating regions of peritoneal epithelium into the 

 subjacent mesenchyma of the genital fold (beginning about the sixth 

 day of incubation), and that the sex-cells which have migrated from 

 extra-regional areas to the peritoneal epithelium become involved in 



