228 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



but for the present I will describe the process where it can be seen best in the living 

 chick that is, in the posterior part of the area pellucida. This will become clear 

 by comparing figures 4 and 5 on plate 1 and all of the figures on plate 2. In this 

 posterior part of the area pellucida the angioblasts do not begin quite as early as in 

 the more anterior part, but they are always more massive. Thus in figure 4, plate 

 1, there are a few small, isolated clumps of angioblasts in the anterior part of the 

 area pellucida, while in figure 5 of the same plate there is a definite plexus of them 

 in the posterior part. The characteristics of this plexus are best shown on plate 6, 

 figure 27, a drawing of the area included in the square on plate 1, figure 5. 



In the living chick the bands of angioblasts are very readily distinguished from 

 the network of the coelom because they are much more refractive than the undiffer- 

 entiated mesoderm. This refractivity is much increased during the period of cell 

 division, so occasionally when one first takes out a blastoderm the entire network 

 of angioblasts will appear very brilliant; or again, bands of angioblasts, which are 

 at first rather dull, will gradually develop a high refractivity. This refractivity is 

 due to a change in the cytoplasm of cells which precedes the division of nuclei by 

 perhaps an hour or more. It is characteristic also of mesenchyme cells, as has been 

 described by W. H. and M. R. Lewis, but the phenomenon becomes even more 

 striking when one is dealing with such massive structures as the angioblasts. Since 

 all of the angioblasts of an area such as the posterior zone of the area pellucida pass 

 into this refractive phase at the same time, a living specimen in which the angio- 

 blasts are about to divide becomes a very brilliant object. Besides a greater refrac- 

 tivity, angioblasts have a denser cytoplasm than the mesoderm. This is due to 

 large numbers of granules in the cytoplasm. These granules are the azur-granules 

 which have been brought out by Maximow as characteristic of young red blood- 

 cells. Due to their presence, the cytoplasm of the angioblasts is very dense in the 

 living form and stains intensely in fixed specimens with all of the basic dyes like 

 hematoxylin. This reaction to dyes is also intensified at the time of division. 



Besides this, there is another very important characteristic of the angioblast, 

 namely: the behavior of the cells after division. In watching the living specimen 

 one selects the bands about to divide by the high refractivity of the cytoplasm and 

 then in about an hour the nuclear figures begin to appear, one after another, in 

 quick succession. The only nuclear figure that can be made out in these masses in 

 the living specimen is the metaphase. After the cells have divided the difference 

 between the mesenchyme and the angioblasts is very striking. The mesenchyme 

 cells become excessively irregular, then separate, and the delicate strands of 

 their cytoplasm are reproduced; while the two angioblasts stay together, forming 

 solid masses in which no cell outlines can be discerned in the living specimen. A 

 single, well-differentiated, resting angioblast can be distinguished from mesenchyme. 

 One can not always be sure of a single cell in process of division because both mesen- 

 chyme cells and the angioblasts are highly refractive at that time; but two angio- 

 blasts can always be recognized by this characteristic formation of apparently 

 syncytial masses. Such a clump of two cells is shown in figure 26, plate 6. 



