344 Papers from the Department of Marine Biology. 



DESCRIPTION OF ILLUSTRATIONS. 



Fig. 1. — Composite drawing of left side of mesentery near its root in the 11-day embryo. All 

 the cells shown are from the immediate vicinity of the same section. M, mesothelium of 

 mesentery; h and e, hemoblast and erj^throcyte, respectively, in the aorta; b, eosinophil 

 blood granulocyte, lying among the mesenchymal cells; a, typical primordial germ-cells 

 (primary genital cells), laden with yolk-globules; i, attraction sphere, shown both in the 

 granulocyte and the genital cell. Similar germ-cells appear also in this and immediate 

 sections in the gut (both among the entodermal cells and in the enveloping mesenchyma) 

 and among the mesothelial cells of both the mesentery and the genital ridge. This section 

 is very similar to that of figure 2 (plate V). Flemming fixation, iron-hematoxylin stain; 

 magnified 1,000 diameters. 



Fig. 2.— Portion of genital ridge of same 11-day embryo, showing a germ-cell among the flattened 

 cells of the peritoneal epithelium; the germ-cell contains many mitochondria, of granular, 

 rod, beaded-rod, and a few of filamentous forms; also some yolk-granules, b, an eosinophil 

 granulocyte. X 1,000. 



Fig. 3. — Section through ccelomic angle of same 11-day embryo, showing a germ-cell among the 

 peritoneal epithelial-cells of both the mesentery (aa) and the genital ridge (ac) ; the germ-cell 

 ab lies among the mesenchymal cells of the mesentery, apparently in process of migration into 

 the mesothelial layer. Both aa and ac also are in amoeboid activity. Cell ac is loaded 

 with an apparently continuous mass of yolk material. Magnified 1,000 diameters. 



Fig. 4. — Germ-cells from the 10-day embryo (similar to fig. 2, plate '\'), in the region of the 

 hind-gut. (Helly fixation, Giemsa stain.) a, germ-cell from the entoderm of the gut (the 

 niicleolus stains a deep violet, the nucleus, with its granular reticulum stains a light bluish- 

 pink, and the reticular vacuolated cytoplasm stains a pink color) ; b, germ-cell in mesenchymal 

 layer of gut; c, germ-cell in the peritoneal epithelium of the genital ridge; d and e, germ-cells 

 from the mesenchyma of the developing genital gland;/, eosinophil from mesentery (granules 

 deep red, nucleolus blue). X 1,000. 



Fig. 5. — Erj^throblast within a blood-vessel, in the 11-day embrj'o. X 1,000. 



Fig. 6.— Germ-cells from 12-day embryo. (Helly fixation; iron-hematoxylin stain.) a, germ- 

 cell from among the entodermal cells of the still open portion of the hind-gut (fig. 1, plate VI) ; 

 I, attraction sphere; b, germ-cell from among the peritoneal epithelium of the genital ridge 

 (the nucleolus is stained a deep black, the granular nucleus a brownish gray, the vacuolated 

 cytoplasm a light gray) ; c, germ-cell from middle of mesentery closely enveloped by flattened 

 mesenchymal cells, simulating a close-fitting capsule. X 1,000. 



Fig. 7. — Germ-cell from "germinal epithelium" of genital-gland of 22-day embryo. X 1,500. 

 Helly fixation; stained in loto with Delafield's hematoxylin. The nucleolus stains a deep blue, 

 the granular nucleus hght blue, and the reticluar cytoplasm a still lighter blue. 



Fig. S. — Germ-cell from suh)epithelial mesenchj^ma of genital gland of 2o-day embryo. The 

 cell is closely enveloped by flattened mesenchymal cells, simulating a capsule. X 1,500. 



Fig. 9. — Germ-cell from submesothelial mesenchyma of sexual gland of 32-day embryo. X 1,500. 



Fig. 10. — Germ-cells and blood-cells from the 8-day embryo, o, germ-cell from the splanchnic 

 layer of lateral plate of mesoderm, near the border of the area pellucida; b, germ-cell from 

 mesentery closely enveloped by flattened mesenchymal cells; c and d, hcmoblasts from meso- 

 chyma of mesentery and from the aorta respectively. X 1,500. 



Fig. 11. — Section of entoderm of open gut of 7-day embrj'o from near the border of the area 

 pellucida, showing a germ-cell among the entodermal cells. The cytoplasm of the germ-cell 

 contains large irregular peripheral vacuoles, due to the solution of the yolk-content. A cell 

 from the superjacent mesenchyma is shown at (a) for purpose of comparison between the 

 nuclei of the germ cells, entodermal cells, and more differentiated mesenchymal cells. 

 X 1,500. 



Fig. 12.- — Germ-cell from entoderm of dorsal region of hind-gut, of a 7-day embryo. The 

 cytoplasm contains enormous peripheral vacuoles, where the yolk-globules have been dis- 

 solved. X 1,500. 



Fig. 13. — Germ-cell in the entoderm from about mid-line of area pellucida in a 3-day embryo. 

 The cell is in the act of migrating into the superjacent visceral layer of the lateral plate of 

 mesoderm. X 1,500. The arrow points towards the embryonic axis. 



Fig. 14. — Germ-cell of a 3-day embryo (fig. 1, plate III), from area pellucida near its lateral 

 border. The cytoplasm contains two large vacuoles where some of the yolk material has 

 been dissolved. This cell has a diameter of 16.5 microns. The germ-cells vary from 13 to 

 22 microns in diameter. 



Fig. 15. — Germ-cell from area opaca, near the margin of the area pellucida, of a 3-day embryo. 

 For comparison a nucleus from the entodermal .syncytium is added at a. Note the close 

 similarity of structure. The cytoplasm of the germ-cell contains large spherical yolk-glob- 

 ules, which at this stage of development were not dissolved in the alcohols after Helly fixation 

 (iron-hematoxylin stain). Only at later stages, when the yolk has suffered further changes 

 in the process of elaboration for nutritive purposes, does it become susceptible to the solvent 

 action of the alcohols. X 1,500. 



