ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 251 



undifferentiated zone of axial mesoderm, where vessels are forming. The transi- 

 tion is shown in two drawings, figures 23, plate 5, and figure 26, plate 6. 



I have given two photographs of blastoderms after the circulation has begun. 

 The first is from a chick with 17 somites (figure 12, plate 3), the second from one with 

 18 somites (figure 13, plate 3). Both show that the mass of blood in the outer rim 

 of the area opaca is drawn forward into the veins by the beat of the heart. In 

 figure 12, plate 3, there is a new generation of tiny blood-islands just beginning in 

 this depleted area. Such new islands are also shown in figure 21, plate 5, from a 

 section through the marginal vein in a specimen of the same stage, i. e., 17 somites. 

 They show very interestingly that a new cycle of blood-islands is beginning in the 

 outer area; this means that a wave of blood-islands will again sweep across the 

 area vasculosa. In both of the specimens there are masses of free blood in the inner 

 rim of the area opaca. These are due not alone to an old generation of blood- 

 islands just breaking up, but also to the fact that opposite the omphalo-mesenteric 

 arteries, which are clearly seen in the older specimen (figure 13, plate 3), there is 

 some abnormal heaping up of red cells in these chicks that are grown on cover-slips. 

 Both specimens show that now the posterior zone of the area pellucida has vessels 

 throughout the outer rim, the new angioblasts being in the axial zone opposite the 

 undifferentiated mesoderm. These new angioblasts belong to the posterior end of 

 the aorta. In the vessels that lie in the arch of the area pellucida posterior to the 

 omphalo-mesenteric arteries are to be seen the blood-islands. Figure 13, plate 3, 

 shows about the maximum number of islands which I have found in this area. 

 This blastoderm has certain especially interesting points: (1) That there are numer- 

 ous islands in the arteries in direct line with the active circulation, and (2) that in 

 this particular specimen the zone of new islands extends far forward into the venous 

 portion of the area pellucida. 



From the figures representing stages with 11, 14, 17, and 18 somites, it is clear, 

 I think, that any section through the lower end of the spinal cord in any of these 

 stages would show all of the processes by which blood-vessels and blood-cells are 

 formed. Thus, if a section were taken through the lower end of the central nervous 

 system in figure 12, plate 3, one would see three generations of blood-islands. In 

 the vessels of the marginal sinus one would find little new islands with two or three 

 cells, like those in figure 21, plate 5; in the inner rim of the area opaca old islands 

 just about to break up, and in the area pellucida islands intermediate between 

 these two extremes. Thus it is clear that after the vessels have formed in the outer 

 part of the posterior zone of the area pellucida, as at the stage of 17 somites, one 

 wave after another of blood-islands sweeps across the area vasculosa, beginning 

 with the outer margin, so that one can find zones of young islands alternating with 

 zones of old ones. In one of my specimens, in which one of these cycles of blood 

 formation was just beginning, so many new unicellular islands were forming in the 

 area pellucida that it seemed almost as if there was an entire duplication of the 

 endothelium. Again, in some of the specimens of 17 somites a cycle of the islands 

 has just been completed and the walls of the vessels are almost bare of islands. If 

 one considers the posterior half of the area vasculosa during the second day of incu- 



