GERM-CELL ORIGIN IN VERTEBRATES.
The detailed and critical study of the origin of the germ-cells in vertebrates may be said to have begun with the observations of Hoffman (1893) on certain birds (including Gallinula chloropus, Sterna paradisea, and Hcemotopus ostralegus), which disclosed primordial germ cells in the entoderm and splanchnic mesoderm far removed from the site of the future gonads, and before a "germinal epithelium" had developed (23-somite stage).
The work of Eigenmann (1892, 1897) marked an epoch in this line of investigation. He clearly demonstrated the extra-regional origin of primordial germ-cells in the viviparous teleost Cymatogaster and showed that they closely approximated the size and appearance of the fifth-cleavage blastomeres, on the basis of which he argued that the germ-cells were segregated at the fifth-cleavage stage. An extra regional entodermal origin of primordial germ-cells has been described also by Dodds (1910) in the teleost Lophius.
In 1904 D'Hollander described the origin of the germ-cells (oogonia) in the chick embryo of the tenth day from epithelial buds of the peri toneal epithelium, in conformity with the earlier views of Waldeyer. This discrepant observation is now elucidated by the recent works of Swift (1915, 1916), which show that such buds do indeed occur, but that they include, but do not originate, primordial germ-cells. The latter have an extra-regional origin.
Beard in 1904 described the extra-regional origin of primordial germ cells also in the skate Raja basis. These cells are said to arise in the anterior portion of the embryonic shield and to migrate along a very definite route to the gut entoderm, thence through the splanchnic mesoderm to the future gonad. A very similar later history has been described by Woods (1902) also for the dog-fish, Squalus =Whigs.
Beard and Eigenmann agree that the germ-cells do not divide for a long period in the early development of the embryo, that the original number remains constant except for a slight diminution due to degen eration, and that a few may become stranded in the migratory process in locations outside of the genital glands.
The most detailed and satisfactory study of the germ-cells in the chick has been made by Swift (1914, 1915, 1916) with the aid of mitochondria) technics. He describes the origin of the primordial germ-cells in a crescentic antero-lateral area from germ-wall entoderm at the margin of the area pellucida, during the primitive-streak stage and until 3 somites are formed (23-hour stage). The cells by amceboid 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 ccelomic angle. After the 25-somite stage no germ-cells remain in 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 coelomic angle. Here they remain until the formation of the gonad begins, when they gradually pass into that organ and become the ofgonia 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.