Embryonic History of the Germ-Cells of the Loggerhead Turtle. 341 



stimulus, a focus of neoplastic growth. An occasional cell is found also 

 in the blood-vessels of this region. Such may be carried by the blood- 

 stream to distant regions and perhaps again enter the mesenchyma or 

 degenerate within the vessels. 



6. The total number of primordial germ-cells counted in a 12-day 

 embryo is 352, the number within the gonads being about equally 

 divided between the two (118 left to 127 right). 



7. Occasional cells may divide by mitosis, or undergo degeneration, 

 at any stage of their history or at any point of the route. Mitoses are 

 more numerous during earlier stages and among the entodermal cells; 

 degeneration is more general during the later stages and in the mesen- 

 chyma of the closed hind-gut. 



8. No germ-cells were found contributing to the formation of the 

 Wolffian duct. There is no evidence in this form in support of von 

 Berenberg-Gossler's claim, on the basis of his observations on the 

 lizard embryo, that the so-called primordial germ-cells represent 

 simply a belated stage of mesoderm formation from entoderm. 



9. The germ-cells do not differ from young somatic cells in the 

 character of their mitochondrial content. The mitochondria include 

 granular as well as beaded-rod and filamentous forms. 



10. No transition stages between coelomic epithelial cells and germ- 

 cells appear up to the 32-day stage. From the 16-day stage on, when 

 the nuclei of some of the germ-cells within the gonads become coarsely 

 granular and the reticulum stains more deeply, apparent transition 

 stages occur between the larger of the mesenchymal cells and the 

 smaller included subepithelial germ-cells. But no secure histologic 

 basis can here be found for separating the germ-cells of the gonads into 

 large "primary genital cells" and smaller "secondary genital cells" 

 (Felix) or "gonocytes" (Dustin) derived by process of differentiation 

 from the cells of the "germinal" (peritoneal) epithelium or the subjacent 

 mesenchyma. The size variations among the germ-cells of the gonads 

 of the older stages are no greater than in the original cords of the area 

 pellucida or in the subsequent early stages; and the cytologic similarity 

 between the two dimensional grades of cells is much closer than 

 between the larger mesenchymal cells and the smaller germ-cells. 



1 1 . The evidence derived from a study of the Caretta embryos is in 

 complete harmony with the idea of a single uninterrupted line of sex- 

 cells from primordial germ-cells to oogonia and spermogonia, and with 

 the hypothesis of a vertebrate Keimbahn or continuous germinal path. 



12. The variations in the distribution of the primordial germ-cells 

 during earlier embryonic stages described by various investigators for 

 a number of vertebrate forms as pertains both to their presence in 

 blood-vessels (chick, Swift; duck, von Berenberg-Gossler) and in 

 various regions and tissue remote from the more direct and more 

 usual germinal route (Wolffian duct and somatopleure in the lizard, 



