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Youngest Embryos from 2 to 5 Days

plate, germ-cells, cells, fig and entodermal

YOUNGEST EMBRYOS; FROM 2 TO 5 DAYS.

Towards the end of the second day of incubation, when 5 somites are formed, the primordial germ-cells are just becoming segregated within the entoderm of the area pelllucida. The cells now occur in two bilateral cords, more or less oval in transection. These cords extend from just in front of the primitive pit (neurenteric canal) to the caudal end of the primitive streak; they lie laterally in the area pel lucida. The germ-cells are not yet as closely aggregated as during the following day. Cells can still be found in process of migration from the yolk-sac entoderm of the area opaca to the entoderm of the area pellucida.

At the 3-day stage (10 somites) the cords of primordial germ-cells are more closely segregated and have apparently moved slightly more medially (fig. 1, plate 3). They now extend from the sixth somite to the caudal extremity of the primitive streak. These cords do not have a uniform diameter throughout, but are more or less interrupted, suggesting a segmental condition, recalling the "gonotome theory" of Ruckert.

Figure 15, plate 2, shows a germ-cell from among the yolk-laden entodermal cells of the area opaca of the 3-day stage; an entoderm cell nucleus is added at the right (a) to show the similarity of nuclear condition between the germ and entodermal cells. Figure 14, plate 2, shows a germ-cell taken from the area pellucida. This cell has a diameter of 16.5 microns. Figure 13, plate 2, shows a similar cell in process of migration from among the entodermal cells of the area pellucida into the overlying visceral layer of the lateral mesoderm. Closely similar conditions appear also at the 2-day stage.

The 5-day stage (fig. 2, plate 3, and fig. 1, plate 4) represents a crucial epoch in the early history of the primordial germ-cells. The gut is open throughout and the embryo lies flat upon the yolk. The

entodermal cords of germ-cells have made an intimate linear contact with the visceral plate of the lateral mesoderm (fig. 1, plate 4), and are beginning to pass into the latter and to migrate medially. The 4-day stage shows transition conditions between those described for the 3 and 5 day stages. Meanwhile the angioblast layer of the visceral meso derm is becoming vascularized, and an occasional cell may become in closed by, or migrate into, a blood-channel, from which it again migrates into the surrounding mesenchyma or possibly later degenerates some where in the blood-vessels.

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.