ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 231 



the phenomenon of cell division. It can readily be seen that there must be a stage 

 in the differentiation of a single angioblast from mesoderm when it is difficult to 

 determine the fate of a given cell. This point can be seen in almost any chick of 

 about 11 somites in the posterior part of the area pellucida, where there is a very 

 active zone of the differentiation of new angioblasts (figs. 8 and 9, plate 2). Thus, 

 for example, on the right side of figure 24, plate 5, there is a single cell which, while 

 not quite so granular as the main mass of angioblasts, still looks more like an angio- 

 blast than like a mesodermal cell. I interpret this cell to be an angioblast about to 

 join the main mass. The same point can be seen in figure 27, plate 6, where there 

 may be some difficulty in determining the actual fate of some of the cells that con- 

 nect the angioblasts with the mesoderm beneath. As soon as a cell is completely 

 differentiated into an angioblast, and certainly after its first division, there is no 

 difficulty in identification. For example, the small clump of cells ventral to the 

 mesoderm shown in Van der Stricht's figure 2 (1895, page 209), from the blastoderm 

 of a rabbit, can with assurance be identified as a group of angioblasts, just as there 

 is no question of the fate of the clump of two cells shown in my figure 26, plate 6. 



METHOD OF FORMATION OF BLOOD-VESSELS FROM SOLID ANGIOBLASTS. 



The next stage in the development of blood-vessels is the formation of a lumen 

 in these solid bands of angioblasts. In his original description of the development 

 of blood-vessels His (1868) asserts that they are at first solid and then, by some 

 method which he could not make out, acquire a lumen; but that after the lumen is 

 formed the resulting endothelial cells are less granular than the original solid masses. 

 Shortly afterward, in 1871, this process was correctly analyzed by Klein. This 

 work I am quoting from the Jahresberichte of 1872, since the original was not 

 accessible. Klein states that certain cells of the deeper layers of the mesoderm 

 become hollow through vacuolization. Through the enlargement of these vacuoles 

 vesicles are formed, the walls of which become endothelium and then, by subse- 

 quent division of the endothelium, there develop within the vesicles masses of 

 cells, partly colored yellow with hemoglobin and partly uncolored. Thus in this 

 early work is given perhaps the best description of the actual processes by which 

 vessels can be seen to form in the living chick embryo. I agree with every one of 

 these points except as regards the early stages ; i.e., that during the second half of the 

 second day of incubation all of the cells attached to the inner wall of the vessels 

 develop hemoglobin. 



All subsequent investigators of blood-vessel development in the chick have 

 described the vacuoles as being seen in the primitive masses of cells which produce 

 blood-vessels; for example, 0. Van der Stricht, Riickert, Mollier, Maximow, Dan- 

 chakoff , and Lillie all have called attention to this phenomenon, but the importance 

 of the process first described by Klein is now brought out convincingly in these 

 observations on the living form. If one watches such a band of angioblasts as that 

 seen in figure 20, plate 4, vacuoles will begin to appear in the solid mass, often just 

 under the edge, but many times directly in the center. It is a very striking process 

 in the living chick and I endeavored to obtain drawings of it, but the changes in 



