YOUNGER EMBRYOS; FROM 9 TO 6 DAYS.
The 9-day embryo (figs. 1 and 2, plate 5) differs from the 10-day embryo only in the relative abundance of the germ-cells at different levels of the migration route; the germ-cells are relatively less abundant in the higher levels and in the gonads.
In figure 10a, plate 2, is illustrated a germ-cell of an 8-day embryo taken from the splanchnic mesoderm at about the mid-region of the area pellucida. It appears to lie in a lacuna, the product of shrinkage; b is taken from the mesentery and is closely enveloped by mesenchymal cells. The latter germ-cell has a diameter of only about 13 microns; a, about 16 microns. For comparison, two hemoblasts are included, c and d. The germ-cells at this stage are most abundant in the mesen tery and in the median portion of the splanchnopleure.
In the 7-day embryo the hind-gut is open throughout—that is, beyond the anterior tip of the pronephros. The mesentery is relatively very long caudally. Some germ-cells are found among the entodermal cells medially, but the majority are in the long mesentery, widely scattered, both among the mesenchymal and the mesothelial cells, and in the medial extremity of the visceral plate of mesoderm. A few cells are already found within the genital ridge and a few remain scattered among the entodermal cells of the open gut. In figure 12, plate 2, is illustrated a large germ-cell from among the entodermal cells of the hind-gut, at the point of initial closure. The cytoplasm contains
enormous peripheral vacuoles and is much shrunken, due to the dis solution of the yolk-masses of the cell. Figure 11, plate 2, shows a younger germ-cell among the entoderm-cells of the area pellucida, near the mid-line. The difference in size of these two cells (16 microns and 20 microns) might be interpreted to indicate a considerable growth of the germ-cells at about the 7-day stage, but it more probably indicates merely a normal size variation, since large germ-cells appear already at the 3-day stage (10 somites) (figs. 13, 14, and 15, plate 2). Swift (1914) records a variation in diameter of the primordial germ-cells in the chick embryo of from 14 to 22 microns, the average being about 16 microns. In this embryo a few germ-cells among the entodermal cells of the open gut and in the mesothelium of the mesentery are in mitosis.
The body of the 6-day embryo lies almost flat upon the yolk ; the gut is open behind the cephalic extremity of the pronephros. Only at this point is there a mesentery, and it is very short and contains no germ-cells. The germ-cells occur in both the entoderm and mesoderm of the open gut, the majority lying scattered in the mesoderm medially.