No attempt is made by von Berenberg-Gossler to apply the above interpretation to conditions in the chick, where these cells during early stages have an intravascular distribution. However, he expresses doubt regarding the germ-cell nature of these intravascular "germ cells," many of which are in process of degeneration. A common interpretation of the phenomena relating to these " entodermal wan 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 amceboid 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 the sex-cords, where as oOgonia and spermogonia they undergo pro liferation and further differentiation.
Considering all the forms studied, from Ascaris to man, the evidence is all but complete for a morphologic continuity of the germ-cells, a "Keimbahn," or germinal path. According to Buchner (1910) no evi dence appears for such a germinal path in annelids, mollusks, and echino derms, nor according to Fick (1906) for plants. But these are simply instances regarding which our evidence is as yet very incomplete.
The evidence derived from this study of the embryo of the loggerhead turtle supports the theory of extra-regional origin of germ-cells and, so far as it goes, the Keimbahn theory. The facts are most closely in line with those reported by Allen for Chrysemys, by Woods for the dog-fish, and by Beard for the skate.
This work had in view three chief objects: (1) to discover the origin of the germ-cells and the route and manner of possible migration; (2) a careful examination of the cellular contents of all blood-channels with the expectation of removing a possible confusion between germ cells and certain blood-cells; (3) to find a basis for harmonizing the results of von Berenberg-Gossler in the case of Lacerta with those reported for other reptiles, and possibly other vertebrate forms.