230 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



that it was only necessary to wait until the phase of division was over, when the 

 endoderm would again clear up. In the specimen from which figure 3, plate 1, is 

 taken, practically every cell of the endoderm has a nuclear figure. 



As can be seen in figure 20, plate 4, the nuclei in the living, resting cells can not 

 be made out in these dense bands. The structure has the appearance of a syn- 

 cytium; there is some network, occasional globules of yolk, and granules in the 

 cytoplasm. The granules I take to be the basophilic granules and the mitochondria. 

 The only time when these bands of angioblasts look as if they were made up of 

 individual cells is when they are passing through the stage of cell division. During 

 this time, both in the living blastoderm and in fixed specimens, each nucleus is 

 surrounded by its own zone of cytoplasm, as is clearly shown in figure 24, plate 5, 

 from a fixed specimen. This chick had 10 somites when taken from the shell, and 

 the heart was just beginning to beat; when fixed it had 11 somites. The heart 

 stopped beating after 1| hours, but the embryo lived and was fixed 2 hours later 

 during a cycle of cell division. The blastoderm is shown in figure 8, plate 2, and 

 the area just referred to (fig. 24, plate 5) is within the rectangle drawn on the 

 photograph. The drawing is reversed in position from the photograph. This 

 specimen is one in which the cytoplasm shows the changes of cell division and 

 brings out the fact that after the cytoplasmic period of preparation all the nuclei do 

 not divide at exactly the same moment; hence some of the nuclei are in the prophase, 

 others in the metaphase, and a few in the anaphase. In the living specimen it can be 

 seen that one cell after another divides until all have divided at the end of the cycle. 



In figure 24, plate 5, it is clear that the band is still solid; the process of lique- 

 faction (which will be described in the next section) has not yet begun, and after 

 all nuclei have divided the band will again look like syncytium as complete as that 

 shown in figure 20, plate 4. This is the more certain in that the specimen from 

 which figure 24, plate 5, is taken has 11 somites and is thus at a stage when the 

 angioblasts of the posterior rim of the area pellucida are in process of growth rather 

 than in the stage of transformation into vessels. The earliest specimen in which I 

 have seen the formation of a lumen in this posterior zone was one of 12 somites; 

 usually, the condition is just beginning at the stage of 13 somites. 



It will be noticed in figure 24, plate 5, that along the lower left margin there is 

 a nucleus which has elongated slightly and looks as if it had undergone a slight 

 differentiation into the type of nucleus characteristic of endothelial cells. When 

 the vacuolation takes place there is a real differentiation of the cells along the 

 border into endothelium, both as regards the nuclei and the cytoplasm; that is, the 

 nuclei elongate and the cytoplasm becomes less granular. If one did not know the 

 actual history of a specimen such as is shown in figure 24, plate 5, one might inter- 

 pret its appearance as evidence that vessels form from angioblasts by a breaking 

 apart of the individual cells of the solid mass, according to the generally accepted 

 idea; but I am convinced from a study of the living form that the essential points in 

 the process of vessel formation are a differentiation of the cells on the margin into 

 endothehum and a liquefaction of part of the original protoplasm of the mass to 

 make blood-plasma, and that this rounding up of the cells of the mass is a part of 



