EMBRYONIC HISTORY OF THE GERM-CELLS OF THE 

 LOGGERHEAD TURTLE (CARETTA CARETTA). 



By H. E. Jordan. 



INTRODUCTION. 



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 coelomic 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- 

 meric 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." 



Riickert 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 coelomic 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 entoderm in a peripheral crescentic area in the proamniotic 

 region of the primitive-streak stage, from whence they are described 



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