324 Papers from the Department of Marine Biology. 



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- 



