LIST OF ILLUSTRATIONS. 261 



PLATE 4 continued. 



FIG. 19. Blood-islands from the area pellucida of a chick (No. 135) with 19 somites, drawn to show the appearance 

 of the living specimen. The chick was incubated for 42 hours and 20 minutes in Locke-Lewis solution having 

 1.07 per cent of NaCl. The heart was beating and the circulation was established. The islands were taken 

 from the posterior part of the area pellucida at about the same region as the drawing of figure 18 of the same 

 plate. The specimen shows a late stage in the development of the islands; the cell outlines are an indication 

 that the islands were undergoing division, though the chromosomes were not seen in the living specimen. 

 In the fixed specimen more than half of the nuclei are in the prophase, which can not be made out in these 

 thick specimens while living. X450. B. i., blood islands; e., endothelium; i, interspace; I, lumen of the 

 plexus of vessels. 



FIG. 20. Plexus of angioblasts from a chick (No. 161) with 13 somites, drawn to show the appearance of the living 

 specimen. The chick had been incubated for 50 hours and then grown for 3 hours and 45 minutes in Locke- 

 Lewis solution in which there was 1.06 per cent of NaCl. It had 12 somites when the egg was taken from 

 the shell and the drawing was made at that stage. The heart was beating, but there was no circulation. 

 The drawing is to be compared with the blastoderms shown on plate 2, figures 8 and 9, and on plate 3, figure 

 10; and is from the zone just lateral to the rectangle shown on plate 2, figure 8. The drawing shows the 

 character of the bands of angioblasts before there is any liquefaction whatever as seen seen in the living chick. 

 X475. A, angioblasts; i, interspace. 



PLATE 5. 



FIG. 21. Section through the marginal sinus of a chick (No. 96) with 18 somites, which had been incubated for 43 

 hours and then grown for 2 hours in Locke-Lewis solution. It had 17 somites when taken from the shell, 

 and when fixed the eighteenth was just appearing. It is to show a new generation of blood-islands beginning 

 in the marginal sinus, like those of the total preparation shown on plate 3, figure 12. The heart was beating 

 and the circulation well established. The ectoderrn is not included in the drawing. X700. B. i., unicellular 

 blood-island in the edge of the marginal sinus; to the left is a larger blood-island, also attached to the endo- 

 thelium, and the sinus has many free erythroblasts; c, exoccelom; en., endoderm; en. c., endodermal cell full 

 of yolk and ready to become a wandering cell; mes., mesoderm along the border of the exocoelom. 



FIG. 22. Drawing of the transition zone between the outer and inner zones of the area opaca in a chick (No. 213) 

 with 5 somites. The total blastoderm from which the drawing was taken is shown on plate 2, figure 6. The 

 place of the drawing is shown by the rectangle in the photograph, but the position is reversed. On the left 

 side, outer zone of the embryo, the vessels have formed and show blood-islands attached to their walls, 

 while on the right side the hollow vessels lead over into the plexus of solid angioblasts. X330. A, angio- 

 blasts, the suggestion of cell outlines indicating cell division; b. i., blood-island within the lumen of the vessel; 

 i, interspace; I, lumen of a vessel. 



FIG. 23. Drawing of a mass of angioblasts in which the liquefaction of the cytoplasm is very extensive, from a chick 

 (No. 174) with 14 somites. It is from the blastoderm shown on plate 3, figure 11. Both this figure and the 

 one on plate 6 (fig. 26) were taken within the rectangle on the photograph. They are both vessels which 

 make up the lower end of the developing aorta. This figure shows a mass of angioblasts which is going to 

 liquefy entirely, while the other shows a partial transformation of angioblasts into erythroblasts. The 

 vacuoles vary greatly in size, and many of them are immediately against the nuclei. X920. 



FIG. 24. Drawing of solid angioblasts from the posterior part of the area pellucida from a chick (No. 177) with 1 1 

 somites. It is from the blastoderm shown on plate 2, figure 8, the position of the drawing being shown by 

 the rectangle on the photograph. The drawing is reversed from the photograph. The angioblasts are 

 entirely solid and show no sign of liquefaction. The mass was undergoing division and different types of 

 nuclear figures are plain. At the lower left-hand border a single nucleus has elongated to make a typical 

 endothelial nucleus, while on the right-hand border a single cell has become an angioblast and is about to 

 join the main mass. X920. 



FIG. 25. Drawing of angioblasts taken during the process of liquefaction from a chick (No. 172) with 13 somites. 

 The specimen had been incubated for 48 hours and 30 minutes in Locke-Lewis solution having 1.06 per cent 

 of NaCl and only 0.014 per cent of CaClz. The drawing is taken from the arch of the area pellucida poste- 

 rior to the end of the spinal cord in a specimen nearly like that on plate 3, figure 10. The chick had 11 

 somites when taken from the shell and the heart was just beating. The entire center of the mass is solid, 

 but there are vacuoles forming along the edges, leaving an endothelial border. In the upper right-hand 

 process the lumen of the vessel is forming within a single angioblast between two nuclei. X920. 



PLATE 6. 



FIG. 26. Drawing of a vessel in which a part of the original angioblasts has developed hemoglobin and is thus forming 

 blood-islands instead of liquefying; from a chick (No. 174) with 14 somites. It is from the blastoderm shown 

 on plate 3, figure 11. Both this figure and that shown on plate 5 (fig. 23) were taken within the rectangle 

 on the photograph. They are both vessels that form a part of the lower end of the developing aorta. The 

 clump of two cells to the right are new angioblasts; in the specimen they are along the vessel wall just out- 

 side of the area drawn, but are about the same distance from the main vessel; they were shifted in the 

 drawing, so as to come within the field. X920. 



FIG. 27. Drawing of a plexus of angioblasts as seen against the developing ccelom in the area pellucida of a chick 

 (No. 198) with 5 somites. It is from the blastoderm shown on plate 1, figure 5, and covers the area of the 

 square on that photograph. The angioblasts are shown as dense bands against the more delicate background 

 of the ccelom. In several places there are transition zones between the two structures. There are numerous 

 gaps in the mesodermal layer. X400. A, angioblasts; mes., mesoderin. 



