250 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



blood-cells in one section. Thus, in the photograph of such a section as that shown 

 in figure 17, it is plain that the vessels of the outer rim, constituting the sinus 

 terminalis, are well formed and contain free blood-cells. Just internal are vessels 

 with blood-islands attached, while in this section the angioblasts just at the border 

 of the area opaca are in process of liquefaction to form vessels. (This zone, outlined 

 in the photograph, is shown in plate 6, fig. 28.) Within the area pellucida are tiny 

 clumps of solid angioblasts that have not yet begun to liquefy. 



Thus we have seen that at the stage of 4 somites the area opaca contains solid 

 masses of angioblasts. At the stages of 5 to 7 somites these angioblasts may be 

 forming vessels in the outer rim of this area, while the stage of from 7 to 11 somites 

 will show this outer rim of vessels filling up with blood-cells and the angioblasts 

 of the inner rim of the area opaca liquefying to form vessels. In the posterior part 

 of the area pellucida, on the other hand, the stages of 5 to 11 somites are marked 

 by the active production of new angioblasts, while at the stages of 6 to 9 somites 

 vessels in the anterior zone can be seen to form from angioblasts. 



The next specimen in the series (fig. 10, plate 3) is from a chick of 14 somites 

 and is evidently quite characteristic for this stage, as it is so much like Lillie's figure 

 45 (1908, p. 88) and Ruckert's figure 886 (1906, p. 1214). This specimen repre- 

 sents a stage before the circulation has begun, although the heart was beating 

 vigorously. The entire area opaca is covered with well-formed vessels. In the outer 

 half the terminal sinus, as can be plainly seen, is quite filled up with free erythro- 

 blasts, more so in my figure than in the other two. The blood in this peripheral 

 zone is now ready to be moved forward into the venous end of the area vasculosa, 

 which is indeed the first movement of the blood after the circulation begins. The 

 inner margin of the area opaca, on the other hand, shows vessels well formed and 

 practically devoid "of free erythroblasts, but with numerous young, attached blood- 

 islands. The actual vessels show better in figure 11, plate 3, from another chick of 

 the same stage. They also show in the same manner in the drawings given by 

 Riickert (1906, figure 886,) and Lillie (1908, figure 45), where the interspaces are 

 pale rings and the vessels are a gray network. This is just the way the vessels 

 themselves appear in the specimens, and at first sight almost everyone takes the 

 interspaces to be the lumina of the vessels not only in a living blastoderm, but 

 also in the fixed specimen. Having once seen the blood circulating in the vessels, 

 however, one will not be confused on this point. Clinging to the walls of the vessels 

 and projecting into the lumina are numerous small islands. In all four figures these 

 extend well into the region opposite the venous end of the heart. 



In figure 10, plate 3, throughout the area pellucida the vessels are well formed 

 except in the posterior region, where they are still solid angioblasts; this is also true 

 of Lillie's and Ruckert's figures just mentioned. In none of the three figures are 

 these angioblasts conspicuous, a fact which I interpret to mean that all three speci- 

 mens happened to be fixed during the resting phase. In my own, I know this to be 

 the case. In this specimen liquefaction is just beginning in the margin of the pos- 

 terior rim between the area pellucida and the area opaca. In figure 11, plate 3, 

 on the other hand, this posterior zone has well-formed vessels except over the 



