EMBRYONIC HISTORY OF THE GERM-CELLS OF THE LOGGERHEAD TURTLE (CARETTA CARETTA).
The wide discrepancies in the published accounts of the origin and early history of the germ-cells in vertebrates provided the stimulus for the inception and prosecution of this investigation. Thus Wal deyer (1870) described in the 4-day chick embryo the differentiation of germ-cells from the mesothelium covering the mesonephros. This portion of the ccelomic epithelium he designated "germinal epithelium." The theory of germ-cell origin from the mesothelium of the genital ridge early received the indorsement of Semper (1875). Semon (1887) also claimed to have seen a number of cells undergoing metamorphosis from "germinal epithelium" into germ-cells. This theory still forms the basis of the almost universal text-book account, and claims adher ents among very recent investigators, e. g., Gatenby (1916).
In 1880, Nussbaum, on the basis of observations on trout and frog embryos, advanced a rival theory which maintained a direct blasto merle origin of the germ-cells and an extra-regional segregation until relatively late stages in the histogenesis of the sexual gland. The central idea of this theory was later (1886) generalized in Weismann's hypothesis of "the continuity of the germ-plasm." Ruckert in 1888 described the origin of the "primordial germ-cells" from a portion of the segmental mesoderm, the so-called "gonotome." Minot (1894) analyzed the available evidence, but adjudged it inade quate to support this novel hypothesis.
In 1892 Hoffman published new evidence, derived from a study of the embryos of various birds, in support of Nussbaum's theory of a germinal path ("Keimbahn") in vertebrates. He developed further the idea that the germ-cells are early segregated as independent ele ments, sui generis, and that these find their way from among the cells of the entoderm to the ccelomic epithelium covering the genital ridge. An increasing number of investigators (e. g., Eigenmann, 1892; Woods, 1902; Allen, 1906, 1907, 1911; Swift, 1914, 1915, 1916) are adding evidence in extension of Hoffman's observations and in accord with the segregation theory of Nussbaum.
In the chick, Swift (1914) traces the origin of the germ-cells back to the entodermr in a peripheral crescentic area in the proamniotic region of the primitive-streak stage, from whence they are described as migrating by way of developing blood-channels to the splanchno pleure and thence, by amceboid activity, to the primitive gonads.
In the lizard Lacerta agilie, von Berenberg-Gossler (1914) follows the origin of the so-called "primordial germ-cells" ("entodermal wandering cells"—Danchakoff, 1908) to the entoderm of the open gut; but he denies for them a direct genetic relationship to the definitive germ-cells, and regards the whole process as a late phase of mesoderm derivation from entoderm contributing largely to the formation of the mesonephric (Wolffian) duct.
Still others (e. g., Felix, 1906, 1911; Dustin, 1910; Firket, 1914) admit the occurrence of primordial germ-cells in the entoderm and their migration in part to the developing genital gland, but attribute to this process only a phylogenetic significance, an ontogenetic remi niscence of an earlier phylogenetic experience; and they claim that these primitive sexual elements early degenerate and are replaced by secondary sex-cells which are differentiated from the mesothelium of the genital ridge.
In view of these discordant conclusions it seemed desirable that the scope of these investigations should be extended to include many more vertebrate forms and additional workers. Among the whole group of investigators Swift was the first to describe, or even to suggest, a vascular route of early migration. Von Berenberg-Gossler (1914) confirms this point for chick and for duck. The possibility seemed to remain, however, that these intravascular so-called genital-cells might indeed be only a type of blood-cell progenitor, perhaps a hypertro phied hemoblast. With this possibility in mind, suggested also by Minot's earlier tentative interpretation (1894) of these cells as ele ments enlarged in preparation for mitosis, and the later claim of Wini wafter and Sainmont (1909) that they are hypertrophied mesoderm cells, the work on the loggerhead turtle embryos was planned.