Embryonic History of the Germ-Cells of the Loggerhead Turtle. 321 



Already, from a study of only this stage of development, much 

 information can be gained concerning the fact, route, and method of 

 the migration of the germ-cells from the gut entoderm to the peritoneal 

 epithelium of the gonad. It may be stated here that at no stages 

 studied was any primordial germ-cell, with two exceptions, ever seen 

 in a blood-vessel. The blood-channels in this form evidently do not 

 greatly assist in the transfer of the germ-cells to the sex-gland, contrary 

 to what Swift has demonstrated for the chick; nor has such a route 

 been advocated for any other form except duck (von Berenberg- 

 Gossler) ; but that the cells do migrate is indicated by the different 

 locations occupied, progressively nearer the genital ridge, corresponding 

 with successively later developmental stages, and that the migration 

 is the result of an inherent amoeboid capacity is shown by the various 

 shapes assumed by these cells (figs. 3aa, 3afr, plate 1); and the path 

 of migration is quite clearly indicated by the pointed condition of the 

 forward end, combined with the altered (flattened and distorted) 

 condition of the cells lying directly in the path of progression. For 

 instance, the primordial germ-cells among the entodermal cells of the 

 closed hind-gut in the 11-day embryo are surrounded by entodermal 

 cells which have become crowded, flattened, and distorted on either 

 side as if through pressure by a cell which enlarged and assumed a 

 spherical shape in situ. The similarity in structure and staining reac- 

 tion between the germ-cells and the entodermal cells, a similarity 

 which becomes progressively closer as earlier stages are approached, 

 seems to speak in favor of an entodermal origin of the germ-cells or a 

 close genetic relationship between entoderm and germ-cells. When 

 the germ-cell lies near the basement membrane of the primitive gut 

 entoderm, the cellular distortion is such as would be caused by a cell 

 migrating from the entoderm into the surrounding mesenchyma. 

 Occasionally a germ-cell can be seen partially within the entoderm 

 and partially within the mesenchyma, an observation which proves 

 a migratory process; and the manner of the distortion of the mesen- 

 chymal cells shows that this migration is in a peripheral direction. 



Within the mesenchyma of the gut and the mesentery the distortion 

 and flattening of the cells are generally in the direction of the root of 

 the mesentery, thus showing a migratory progression in that direction. 

 Occasional cells, judged by this criterion, are moving towards the 

 mesothelial layer of the mesentery (figs. 3aa, Sab, plate 1). Occasional 

 cells are also found among the cells of the peritoneal epithelium of the 

 genital ridge. In the region of the angle between the root of the 

 mesentery and the gonad, and again within the mesenchyma of the 

 sexual gland, the shape of cell and the distortion of the adjacent mesen- 

 chymal nuclei are such as to indicate a ventro-medial progression to 

 the surface of the gonad, where the germ-cells come to rest among the 

 peritoneal cells (fig. 2, plate 1). Figure 3ac, plate 1, shows a germ-cell 

 similarly located, but almost completely filled with a huge yolk-mass. 



