3. The germ-cells migrate by amceboid activity, assisted in small part probably by the factor of unequal growth, involving the shifting of the medial portion of the splanchnopleure to the mesentery and the dorsal portion of the mesentery to the gonads.
4. The migration period is not sharply limited. It is at its height from the seventh to the twelfth day, and practically ceases about the sixteenth day. But occasional extra-regional cells may still be found in the gut and mesentery at the 32-day stage—usually, however, showing signs of degeneration.
5. A certain number of germ-cells migrate out of the regular germ cell route and go astray. Such strays are especially numerous in the periaortic mesenchyma caudally, where they may become incorporated among the neuroblasts of the developing peripheral sympathetic ganglia. The majority of these strays probably degenerate in situ, but some may possibly persist to form, under the proper pathologic 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 mitochondria) content. The mitochondria include granular as well as beaded-rod and filamentous forms.
10. No transition stages between ccelomic 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.
11. 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, von Berenberg-Gossler ; and sympathetic ganglia in the loggerhead turtle, Jordan) ; and to their apparent primary (urodeles) or secondary (anurans and other vertebrates) derivation from the splanchnic layer of the lateral mesoderm—are incidental to their original location with respect to the embryonic area and the vascularizing mesoblast of the blastoderm, and to their amceboid capacity. Since the primordial germ-cells are genetically directly related to neither of the secondary germ-layers, their origin in either (entoderm or mesoderm) has no fundamental significance. Since they are capable of amceboid activity, and may become included in blood-vessels, they may migrate any where, and so occur in any location, from where they may subsequently migrate again to the more direct germinal path, or perhaps disintegrate. The fact of fundamental significance with respect to the primordial germ-cells is their original extra-regional distribution and their direct genetic independence of the soma-cells.