236 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



the cells which I have found depended so much upon the concentration of the sur- 

 rounding fluid. The angioblast in the second figure shows the beginning of the 

 vacuolation by which the solid masses are converted into vessels. 



ORIGIN OF BLOOD-ISLANDS. 



The angioblasts not only give rise to endothelium and to the blood-plasma, but 

 produce red blood-cells or, strictly speaking, erythroblasts. In the two figures 

 from specimen 174 (figs. 23, plate 5, and fig. 26, plate 6) is shown the striking con- 

 trast between a vessel in which the central mass is going to liquefy entirely and an 

 adjacent vessel in which some of the original mass is going to form a blood-island. 

 These vessels are just along the border of the dense mesoderm of the posterior zone 

 in a chick of 14 somites. Figure 11, plate 3, is a photograph of this blastoderm, and 

 both drawings are taken from within the square on the photograph. The angio- 

 blasts in question are thus going to form a part of the lower end of the aorta. The 

 aorta follows the lateral line of the somites and below the last somite curves out- 

 ward, following the lateral edge of the dense, axial mesoderm. Outside this zone in 

 this particular specimen, there is a zone of blood-islands in the vessels, and mesial 

 to this area masses of angioblasts are becoming vessels, while the masses on the 

 border between the two areas show both processes. In figure 23, plate 5, the central 

 mass is delicate and the vacuolation very extensive, and from observations on the 

 living one would expect that the lumen would be complete in a short time. In the 

 other drawing, on the other hand, a part of the cytoplasm has become much more 

 dense than the original angioblasts. This more deeply staining mass would appear 

 slightly tinged with yellow in the living specimen, due to the presence of hemoglobin 

 in the cells. In other words, it has become a blood-island. I have watched the 

 formation of such a blood-island from the stage of solid angioblasts, as shown in 

 figure 20, plate 4, so that I know it is not to be interpreted as a new outgrowth from 

 endothelium. The blood-island in figure 26, plate 6, is still attached to the wall 

 by guy ropes of the original angioblasts, as well as by a solid base. In this manner 

 exceedingly large and irregular blood-islands form. Such islands are not different 

 in their behavior from those which arise along the walls of empty vessels from a 

 division of the endothelial cells. 



In figure 26, plate 6, there is also another process to be seen, namely, that of 

 differentiation of new angioblasts. To the right of the main vessel is a clump of 

 two angioblasts which have not as yet joined the neighboring vessels nor begun to 

 form a lumen of their own. These two cells were a little farther along the edge of 

 the vessel than is shown in the drawing; they were shifted slightly in the drawing 

 in order to make them come into the same field. Since these cells are going to join 

 the end of the aorta, they are evidence of the fact that the aorta differentiates in situ. 



It is now necessary to describe how blood-islands form in vessels in which there 

 has been a complete solution of the central cytoplasm, without any of the original 

 angioblasts remaining except those that make an endothelial border for the vessel. 

 In watching the living specimen the change in the appearance of the vessels after 

 the lumina have formed is very striking. While they are in the form of solid angio- 



