Others of the more recent workers with chick embryos include Nussbaum (1901), Rubaschkin (1907), and von Berenberg-Gossler (1912). These investigators agree in finding primordial germ-cells in the entoderm and splanchnic mesoderm lateral to the ccelomic angle in embryos of about 22 somites. In a more recent paper von Berenberg Gossler (1915) confirms Swift in his observations that primordial germ cells appear within the vascular channels in chick embryos prior to the 25-somite stage.
Firket (1914) maintains that in birds the majority of the primordial germ-cells disintegrate, though some may possibly come to "maturity" in the wall of the gut; and he views their extra-regional appearance as of only phylogenetic significance, reminiscent of the definitive sex-cells of lower vertebrates.
The work of Allen on the turtle Chrysemytt marginata (1906, 1907) and on the ganoid fishes Amia and Lepidosteus (1911) has greatly advanced our knowledge regarding the origin and development of the primordial germ-cells in vertebrates. In these three forms Allen has likewise demonstrated an extra-regional entodermal origin. In Chrysemys the germ-cells (" sex-cells," Allen) are described as arising from the entoderm near the lateral margin of the area pellucida at a level beginning about the cephalic extremity of the pronephros and extending on either side to a point just behind the caudal tip of the embryo. The total number of the original quota of germ-cells is said to vary from 302 to 1,744, with an average of about 1,100 (Allen, 1907). No mitoses were observed in these cells until the 10-mm. stage of development. During the migration of the sex-cells many are said to lag hopelessly behind, while others may go entirely astray. Upon an average 47.7 per cent are estimated to finally reach the sex-glands, while the remainder are said to come to rest in the alimentary tract, the mesentery, and the region between the root of the mesentery, the aorta, and the mesonephroi.
It is important to note that Dustin (1910) records a much smaller average number of germ-cells in Chrysemys embryos of this stage, the extremes being given as 158 and 415. This number is much closer to my own count (see chart, page 323) for Caretta.
They are described as migratory to a high degree. In Amia they are said to arise from the entoderm of the roof and margin of the floor of the subgerminal cavity.
The history of the germ-cells in Chrysemys is practically identical with that above described for Caretta. However, I find no evidence in Caretta to support Allen's conclusion that " large numbers" of cells fail to migrate eventually to the gonads, but undergo degeneration in these extra-regional locations. Rarity of actual degenerating cells, or persisting representatives of any kind in later stages, indicates an ultimate migration of at least the vast majority of the extra-regional germ-cells into the genital-glands. Moreover, an occasional germ-cell may be seen dividing during all the earlier stages.
Dustin (1907) describes the origin of the sex-cells (" gonocytes") in a 3 mm. Triton larva from the medial portions of the lateral plates of mesoderm in the caudal half of the body. These original sex-cell masses become approximated medially and subsequently fuse to form a longitudinal rod of sex-cells just above the dorsal root of the mesentery. At about the 14 mm. stage the majority of the original sex-cells are said to degenerate and disappear, and a new generation to be differen tiated from the peritoneal epithelium of the genital ridge. A similar mode of origin and history of the germ-cells is described for Rana and Bufo. As regards Rana pipiens, Allen (1907) describes a dorso-medial migration of germ-cells from the gut entoderm at the time when the two lateral plates are approximated in the formation of the mesentery. The initial mesodermal origin described by Dustin is accordingly here an illusion.
In the matter of an entodermal origin of the germ-cells in Rana, Allen has been confirmed by Kuschakewitsch (1908), and by King (1908) as respects Bufo lentiginosus. Miss King finds no evidence in support of a secondary source of germ-cells from the peritoneal epithe lium. Moreover, in Chrysemys, Allen (1906) traced the germ-cells to maturity without finding any evidence of a transformation of peri toneal cells into germ-cells. The same is true also of Caretta at least to the 32-day stage.