ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 233 



During the phase of division the cells of the bands always look as if they were 

 about to separate into individual cells, as suggested in figure 24, plate 5, but if such 

 a specimen be watched until it passes into a resting stage the mass will again take 

 on the appearance of a syncytium. When a single nucleus is caught in the stage 

 of division, like the one in figure 25, plate 5, it looks also as if the cell were to become 

 separated from the mass. 



There is another phenomenon very characteristic of these developing bands of 

 angioblasts which suggests the idea of the breaking apart of the cells to form a 

 vessel, i. e., cell death. Often while the vacuolation is going on, or just before it 

 begins in a mass, occasional cells stand out by virtue of certain special characteris- 

 tics. This is true also of the blood-islands which develop subsequently. These 

 characteristics are, first, that the cytoplasm becomes perfectly clear except for a 

 few irregular masses, a phenomenon very familiar in certain cells in blood-smears; 

 second, the nuclei show a sharp contour which is unusual in a living cell. In the 

 living specimen the dead cells stain with a dilute solution of neutral red, while the 

 living cells do not react at all to the dye. In fixed specimens they show picnotic or 

 fragmented nuclei characteristic of dead cells. 



A still later stage in the process of liquefaction of angioblasts, by which the 

 lumen of vessels is formed, is shown in figure 28, plate 6, a section from a chick of 

 11 somites. Here the process is in the edge of the area opaca since all of the angio- 

 blasts of the posterior part of the area pellucida are still solid at this stage. In this 

 section the cytoplasm just above the label L has almost liquefied, but in the center 

 of the vessel there is still a little clump of cytoplasm labeled A, showing a picnotic 

 nucleus, an indication that there are some degenerative processes going on in the 

 protoplasm. Between the labels A and L in the same vessel the endothelial border 

 itself is very ragged, showing also some degeneration of cytoplasm. 



In certain areas, especially in chicks a little older (for example, during the 

 fourth day of incubation), there are numerous small, completely isolated vesicles, 

 filled with free blood-cells, which look just as if they had become separated from 

 an originally solid mass. Such tiny vesicles are always to be found dorsal to the 

 amnio-cardiac vesicles, opposite the delicate ventral bands of this area which are 

 shown in figure 7, plate 2. This specimen has these small vesicles which are filled 

 with dead cells, as shown by their picnotic nuclei. In other specimens the vesicles 

 are filled with normal red cells, and I believe are formed, like the rest of the blood- 

 vessels, by a partial solution of the center of the mass and fill up subsequently by 

 division of the cells of their walls though there might well be some separating of 

 individual cells in the transformation of the solid center of the angioblasts into the 

 lumen of a vessel. However, in all of the masses of angioblasts which I have seen 

 transformed into a vessel in the living specimen, there has been a considerable 

 amount of the liquefaction of cytoplasm. 



Just as cell division progresses in cycles, so this process of liquefaction pro- 

 gresses by stages, and if one finds it in a single band of angioblasts all of the bands 

 in that area will show the same. In general one can see the process, in any blasto- 

 derm of 6 or 7 somites, taking place in the area pellucida opposite the venous end 



