OLDER EMBRYOS; FROM 13 TO 32 DAYS.
The 13-day embryo is essentially like the one of 12 days (figs. 1, 2, and 3, plate 6), except that relatively fewer germ-cells remain in the gut and more are found within the gonads. In the 16-day embryo (11 mm. in length, from cephalic to caudal bend) the germ-cells have practically all migrated into the gonads, where they come to rest, both among the mesothelial cells and in the subjacent mesenchyma. In the stouter mid-portion of the genital ridge (future gonad), where the mesenchyma is more extensive, the majority of the primordial germ-cells lie subepithelially; in the cephalic and caudal extremities, on the contrary, where the mesenchyma is sparse, the majority of the germ-cells lie in the peritoneal epithelium, having pressed the col umnar cells aside laterally and caused great flattening of the overly ing cells. Only an occasional cell is found within the gut and the mesentery. In three slides, including 60 sections, through the hind gut, three germ-cells were seen in the mesentery (one at the gut end and two in the mid-portion, all apparently in healthy condition), and one in the mesenchyma of the gut. Many of the germ-cells within the gonads at this stage have a coarsely granular nuclear reticulum (fig. 7, plate 2), the granules staining deeply, in contrast to the pale nucleus with very delicate reticulum of the germ-cells of earlier stages.
In figure 7, plate 2, is illustrated a primordial germ-cell from the epithelial layer of the gonad of a 22-day embryo (12 mm. in length). An occasional germ-cell still occurs also in the gut and in the mesentery. The germ-cells of this stage have the same structure and approximately the same size as those from the 10 to 16 day periods, but relatively more cells contain coarsely granular nuclei than in the 16-day stage. The germ-cells of the 32-day stage are no larger than those of the earlier periods, as shown in figure 9, plate 2, of a cell taken from the subepithelial mesenchyma of the gonad. In figure 8, plate 2, is illus trated a germ-cell from the mesenchyma of the mesentery of a 25-day embryo. This particular cell, in common with many others, appears
to be encapsulated by flattened mesenchymal-cells. The flattening of the mesenchymal cells is the result of pressure produced by the migrating germ-cell in the direction of the root of the mesentery. It may be emphasized that the germ-cell has undergone no growth or other striking structural alteration between the tenth and thirty second day of incubation. (Compare figs. 4 and 6, plate 1, with fig. 9, plate 2.) By the sixteenth day all the germ-cells, except a few strays, have left the gut and mesentery and are located within the sexual glands. Also at the 25-day stage an occasional cell may still lag behind in the gut and mesentery; and even in the 32-day embryo several cells were seen in the mesenchyma of the gut, though these were apparently in process of degeneration. The migration process appears to be at its height from the seventh to the sixteenth days; but certain stray cells, mostly degenerating, still persist extra-regionally at the thirty second day.
In a 25-day embryo, fixed in Flemming's fluid and stained with iron hematoxylin, where the germ-cells could be more readily detected because their content of yolk-globules was perfectly preserved, 3 germ cells were seen (in 4 consecutive slides, including 80 sections) in the root of the mesentery—one (degenerating) near the gut end, one in the mid-portion, and one (degenerating) in the mesenchyma of the hind gut ventrally. The germ-cells of the embryo, in contrast to all other cells (except liver-cells), still showed abundant yolk-granules and some mitochondria.
The migration period is obviously not sharply limited, but extends in some degree through half the incubation period. Some germ-cells apparently lag hopelessly behind and degenerate, especially in the gut. The initial conspicuous marks of degeneration relate to stages in the disappearance of the nucleus either by solution or karyorrhexis.