242 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



In spite of the general view that angioblasts differentiate first in the most 

 posterior zone of the area opaca, I am of the opinion that they tend to show first in 

 a more anterior zone, not only in the area pellucida but also in the area opaca. 

 Riickert describes the order as being the reverse in the two areas; that is, first in 

 the posterior part of the area opaca and last in the posterior part of the area pellu- 

 cida. Throughout the early development angioblasts are more massive in the 

 posterior part of both the area opaca and the area pellucida, but in each area they 

 are always a little farther advanced in the more anterior zone. 



At the stage of 2 somites (as shown in fig. 3, plate 1) there is but little change 

 in the mesoderm of the area pellucida, but in the area opaca the great mass of 

 primitive mesoderm has formed large vesicles, the forerunners of the primitive 

 exocoelom. Opposite the middle region of the area opaca, especially, are small 

 masses of dense cells, the first angioblasts. As described by Riickert, these tend to 

 lie opposite the walls of two adjacent vesicles of the exocoelom, so that they alter- 

 nate with its cavities. I have other specimens of 2 somites which show a greater 

 number of angioblasts in this middle zone, but still the same lack of differentiation 

 in the posterior zone. In one specimen of 2 somites I can identify as angioblasts 

 one or two clumps of cells in the area pellucida; again, specimens of 3 somites may 

 how a few angioblasts in the area pelludica, but usually they are not well marked 

 there until the chick has 4 somites, and I believe are never abundant until the stage 

 of 5 somites is reached. 



The stage of 4 somites is especially interesting on account of a differentiation 

 in the zones of the exoccelom. In figure 4, plate 1, from a chick with 4 somites, it 

 will be seen that over the amnio-cardiac vesicles of the area pellucida, and also over 

 the entire anterior half of the area opaca, the layers of cells which make the exoccelom 

 have differentiated into large vesicles. In the posterior half of the area opaca this 

 differentiation is only just beginning, as can be seen in figure 29, plate 6, in which 

 it is clear that along the dorsal border of masses of mesoderm there are two layers 

 of cells, the forerunners of the walls of the exoccelom; while the ventral, solid mass 

 is now made up of angioblasts. Toward the end of the stage of 4 somites this 

 differentiation becomes much more marked. The vesicles of the ccelom become 

 larger and the angioblasts more massive. In the area pellucida the zone of the 

 amino-cardiac vesicles is well marked, as is also the middle zone of the exoccelom, 

 where will be seen the delicate plexus of mesoderm characteristic of this area. In 

 this middle zone are a few angioblasts near the edge of the area opaca. This is the 

 first zone of the area pellucida in which angioblasts appear. The more posterior 

 part of the area shows no differentiation of the mesoderm, except into the more 

 dense axial zone. In the axial line the only indication of the vascular system is a 

 slight thickening of the splanchnic mesoderm which marks the very beginning of the 

 myocardium. 



In the history of the vascular system the stage of 5 somites is very important 

 and is shown in two photographs, figures 5 and 6, the. former being the less devel- 

 oped. On the left side all of the points are obscured by yolk, but the angioblasts 

 are very clear on the right side. The entire area opaca is occupied by two structures, 



