ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 253 



phase; that is to say, all dividing, resting, or undergoing liquefaction at the same 

 time. I can not make this out to be true of the endothelium after the vessels have 

 formed. In other words, there is not sufficient change in the appearance of the 

 cytoplasm of the finished endothelium in the living specimen to indicate whether 

 the cells are going to divide or not, and in fixed specimens one finds only scattered 

 nuclei with mitotic figures. However, a specimen in which there are any endothelial 

 nuclei with figures will show many of them. Therefore, recognition of this process 

 of cycles in cell division depends on finding the changes in the cytoplasm which 

 precede the nuclear changes in the living form. 



That erythroblasts keep on dividing in cycles after they are free from the 

 islands is suggested by identifying several stages of nuclear figures and young cells 

 half the normal size in the circulating blood; that is to say, if one finds one nucleus 

 in the metaphase there will be many in the same phase; or one may find groups in 

 the metaphase and other groups in the prophase. They serve to emphasize the fact 

 that in an embryo after the stage of 5 somites there are two sources of red blood- 

 cells, the endothelial cells of the vessels and free erythroblasts. 



The question as to whether or not the same marked cycles of cell division can 

 be made out for the mesoderm, is an interesting one. In many of the early prepara- 

 tions nearly every cell in the myotomes or very extensive masses of the dense axial 

 mesoderm may be found in division; but I have no specimens proving that the 

 entire mesoderm of the embryo or of the membranes divides at one time, as is the 

 case for the endoderm and the angioblasts. In later stages, when angioblasts are 

 differentiating in large numbers, there may be some difficulty in distinguishing a 

 single angioblast in division from a mesodermal cell in division. This is due to the 

 fact that the cytoplasm of the mesodermal cell rounds up around its nucleus during 

 the phase of division and shows also some increase in density, so that it may simu- 

 late the angioblasts. There is no difficulty in detecting a fully differentiated, resting 

 angioblast, nor any clump of two or more angioblasts. A specimen such as that 

 shown in figure 9, plate 2, for example, has a very large number of single dividing 

 cells near dense clumps of angioblasts, which I interpret as cells which are just 

 becoming angioblasts, though it may be admitted that in watching such a living 

 specimen in which there is a question of the differentiation of large numbers of new 

 unicellular angioblasts, the final proof of the nature of the cells would be their 

 behavior after they had finished their first phase of cell-division. Had the cyto- 

 plasm remained rounded up, and the new cells remained together, they would soon 

 join a neighboring band of angioblasts; on the other hand, had the two cells separ- 

 ated and put out the delicate exoplasm characteristic of mesoderm, the cells would 

 still be undifferentiated. 



A question of great importance is whether or not any red blood-cells can be 

 seen to differentiate outside the lumen of a vessel. All of my evidence tends to 

 show that in these stages the red cells develop only within the vessels. This is in 

 entire agreement with the view of Madame Danchakoff (1909), who has shown that 

 in the chick the development of the red cells is intravascular, while that of the 

 granular, white corpuscles is extra vascular. In the living blastoderm the only 



