Embryonic History of the Germ-Cells of the Loggerhead Turtle. 329 



at the margin of the area pellucida, during the primitive-streak stage 

 and until 3 somites are formed (23-hour stage). The cells by amoeboid 

 activity enter the blood-spaces of the mesoderm, which subsequently 

 grows into this portion of the primitive proamniotic area, and are then 

 carried to all parts of the embryo and the vascular area. The germ- 

 cells remain widely distributed in this way until about the time when 

 the embryo has 20 somites. Numbers of those in vascular channels 

 are said to undergo mitotic division, while some suffer degeneration. 

 The germ-cells remain scattered throughout the blood-vessels until 

 about the 22-somite (44-hour) stage, only a few cells appearing in the 

 splanchnic mesoderm. At the 23 to 25 somite (50-hour) stages the 

 majority of the germ-cells have escaped from the blood-vessels and are 

 now found almost exclusively in the tissue of the splanchnic mesoderm 

 near the coelomic angle. After the 25-somite stage no germ-cells remain 

 hi the blood-vessels. In embryos of from 30 to 33 somites (60 to 72 

 hours) the primordial germ-cells have migrated to the root of the 

 mesentery and into the ccelomic epithelium on both sides of the 

 ccelomic angle. Here they remain until the formation of the gonad 

 begins, when they gradually pass into that organ and become the 

 oogonia and the spermogonia of the ovary and testes respectively. 



Swift describes also a very prominent attraction-sphere, persisting 

 without change from the origin of the germ-cell to its entrance into the 

 gonad, and he uses this mark as the chief character for differentiating 

 between germ-cells and adjacent elements. Such a uniform and 

 conspicuous criterion was lacking in the loggerhead-turtle embryo and 

 could not be employed in the identification of the germ-cells. 



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 Chrysemys 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 



