ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 229 



In figure 27, plate 6, as well as in figure 24, plate 5, can be seen another charac- 

 teristic of angioblasts. Immediately after their differentiation they show a remark- 

 able tendency to send out exceedingly delicate sprouts of cytoplasm toward similar 

 cells. These processes always emanate from a nuclear area, so that when one finds 

 a clump of cells connected with a vessel by slender, solid processes, it is an indica- 

 tion of new cells which have joined the wall. Such sprouts are also shown on the 

 little group of two angioblasts in figure 26, plate 6. By means of this sprouting 

 masses of isolated angioblasts soon form a plexus; thus, even at the stage of 5 somites, 

 at which angioblasts are just differentiating in the area pellucida, there is a very 

 extensive plexus of these bands. In figure 27, plate 6, all of these characteristics 

 are evident. In this specimen the mass of angioblasts showed the protoplasmic 

 changes which precede nuclear division, and the specimen was fixed just as the 

 stage of the metaphase became visible in a few of the nuclei. The bands of angio- 

 blasts thus stand out very clearly against the more delicate network of the ccelom 

 beneath, as is plain in the photograph of this section (plate 1, figure 5). Here and 

 there the bands of angioblasts themselves still show a little of the character of the 

 original mesoderm; that is to say, they are in the process of differentiating. Again, 

 in many places the bands of angioblasts are still connected by bands of cells with the 

 mesoderm beneath, so that the angioblasts fade into the mesoderm, while in other 

 places they are fully formed and fully connected with each other by characteristic, 

 delicate sprouts, figure 27, plate 6. In such a preparation there can be no doubt 

 as to the relation of the angioblasts to the mesoderm, and this relation is as clear 

 in the living specimen as in the fixed preparation. The origin of the angioblasts 

 from mesoderm was clearly brought out by 0. Van der Stricht (1895, page 182) and 

 agrees with the views of Riickert, Mollier, Maximow, Danchakoff, and others. 



The characteristics of the solid bands of angioblasts, as seen in the living form, 

 are brought out in figure 20, plate 4, a drawing of a plexus of angioblasts taken 

 from the posterior part of the area pellucida in a chick of 12 somites, while it was 

 growing on the cover-slip. The position is just lateral to the square on plate 2, 

 figure 8, from a slightly younger specimen. In the living blastoderm the bands of 

 angioblasts appear like a complete syncytium. The interspaces are, of course, 

 indistinct when the angioblasts are in focus, because they represent the layer of 

 endoderm above the angioblasts and the layer of mesoderm beneath. There is 

 never any difficulty in distinguishing the definite layer of endoderm by changing 

 the focus, either in the h'ving specimen or in the total preparations. Under certain 

 conditions, however, the endoderm may prove to be a very confusing factor in these 

 studies. For instance, occasionally the endodermal cells may contain so many 

 droplets of yolk that one can not focus through them clearly. Such specimens 

 never clear up and are useless for a study of the blood-vessels beneath. Or, for 

 some unknown reason, the cells of the endoderm may become excessively vacuolated, 

 in which case they are difficult to focus through, both in the living form and in the 

 fixed specimens. Again, the entire endoderm may divide and its cytoplasm in con- 

 sequence become too opaque to see through. In my early studies I concluded that 

 these specimens had died, but finally saw some of the nuclear figures, and found 



