Embryonic History of the Germ-Cells of the Loggerhead Turtle. 327 



Between the first and sixth days, then, the scattered primordial 

 germ-cells have become segregated from out of the area opaca into a 

 pair of bilateral cords lying near the lateral border of the area pellucida 

 in approximately the caudal half of the embryonic area (fig. 1, plate 3), 

 whence they pass at about the fourth to fifth days into the visceral 

 plate of the lateral mesoderm (fig 1, plate 4), and thence medially 

 towards and into the forming mesentery of the just-closing hind-gut 

 (fig. 1, plate 5). Certain stray germ-cells may persist in any portion 

 of the hind-gut and its mesentery until about the 32-day stage, at 

 which time the final remnant largely degenerates, a few possibly per- 

 sisting and perhaps furnishing foci for future neoplastic growths. 



The very close resemblance between the nuclei of the entodermal 

 cells of the area opaca and the included germ-cells seems to indicate 

 a very close genetic relationship; and their cytomorphic features dis- 

 close an equally undifferentiated condition. The only striking dif- 

 ference between these cells is one of shape; the germ-cell is oval, the 

 entoderm-cell very irregular and apparently in syncytial relationship 

 with other entodermal cells. 



LITERATURE. 

 GERM-CELL ORIGIN IN INVERTEBRATES. 



The clearest and most striking case of early germ-cell differentiation 

 among the invertebrates is that of Ascaris megalocephala, in which 

 Boveri (1892) traced the germ-cells through the cleavage back to the 

 2-cell stage. The conspicuous differential feature of this cell at the 

 2-cell stage (and to the sixth cleavage) is the maintenance at mitosis 

 on the part of one of its daughter-cells of the integrity of the chromo- 

 somes in contrast with the soma-cell, both of whose daughter-cells 

 suffer a diminution of the chromatin at each division. The germ-cells 

 become segregated at the 32-cell stage of cleavage, after which they 

 produce only similar germ-cells at each division. 



Previously Balbiani (1885) had succeeded in tracing the germ-cells 

 (" pole-cells") of the dipter Chironomus to an early stage of differen- 

 tiation in the segmenting egg. ''Pole-cells" were first reported by 

 Robin in 1862 for certain other Diptera, and subsequently (1863) also 

 by Weismann for Chironomus. Hasper has quite recently (1911) 

 identified one of the first four cleavage-cells as the progenitor of all the 

 germ-cells in Chironomus. 



A similar history was traced by Hacker for the germ-cells in Cyclops; 

 here the germ-cell progenitor is recognizable at the 32-cell stage. 



Hegner (1909, 1914, 1915) has worked out very completely the early 

 germ-cell path ("Keimbahn") in certain Diptera and in chrysomelid 

 beetles, where the diagnostic marks are a polar differential granular 

 cytoplasmic content, the "pole-disk," constituting a germ-cell determi- 

 nant. In the dipter Minastor, Hegner (1914) describes a segregation 



