ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



247 



figure 68 B (1908) from a chick of 10 somites, and in Van der Stricht's figure 3 

 (1895, p. 210). In this specimen (fig. 25, plate 5) many of the clumps have become 

 tiny vesicles by the liquefaction of their centers, and several show a few red blood- 

 corpuscles. These vesicles over the somatopleure remain isolated for a long time, 

 simply because they are few and far apart. I have a specimen of the fourth day of 

 incubation, grown on a cover-slip, which has as many as 10 or 12 of them. Some 

 of them, I think, are degenerate, but it soon becomes difficult to follow them in the 

 living specimens on account of the increasing thickness of the blastoderms. Their 

 especial interest lies in noting the early appearance of vessels in the somatopleure 

 in the chick. 



In the middle or venous zone of the area pellucida the angioblasts of this 

 specimen have become vessels. They form a delicate plexus and the vessels are 

 consequently small and inconspicuous in the photograph. In the living chick, how- 

 ever, the process of liquef action can be easily followed. Most of the vessels of this 

 area are empty, that is as far as cells are concerned, but here and there are a few 

 small clumps of red-blood cells, showing that the angioblasts have the potentiality 

 to produce cells bearing hemoglobin. The posterior zone of the area pellucida is 

 especially interesting in this specimen, as it happens to be one in which all of the 

 angioblasts exist in the form of isolated masses of cells. There are numerous deli- 

 cate sprouts from these masses, but for the most part these have not yet joined 

 similar masses of cells. 



M 



TEXT-FIGURE 1. Diagram showing the position of 

 the angioblasts which are the forerunners of the 

 endocardium in a chick (No. 206) with 6 somites, 

 incubated for 24 hours and 30 minutes and then 

 grown in Locke-Lewis solution in which there 

 was 1.05 per cent NaCl. The diagram shows 

 the actual masses of angioblasts making up the 

 endocardium, and is to be compared with the 

 photograph on plate 1, figure o. X135. A, 

 angioblasts of the endocardium; En., line of the 

 endoderm; M., myocardium. 



The stage of 6 somites is the best for studying the differentiation of the heart 

 and aorta from angioblasts. This stage has been extremely well described by 

 Williams (1910-11). Figure 1 in the text shows the edge of the head-fold, and the 

 margin of the myocardium just before it fuses with the symmetrical fold of the 

 opposite side, while between the two are the early chains of angioblasts with tiny 

 vesicles which are destined to make the endocardium of the heart. This figure 

 corresponds to the description of the heart of the Selachian given by 0. Van der 

 Stricht (1896). The endocardium continues to receive new angioblasts from the 

 zone of the myocardium, certainly throughout the second day of incubation. These 

 can readily be seen in total mounts of chicks with 17 to 20 somites spanning the 

 wide gap between the myocardium and the endocardium, which is so characteristic 

 of these early stages. The wide gap is not due to skrinkage, because it can be easily 



