THE DEVELOPMENT OF THE VASCULAR SYSTEM 



269 



There is a view that both the blood cells and the endothelium of blood 

 vessels arise from certain mesamceboid cells of entodermal origin, which are 

 insinuated between the entoderm and mesoderm but are not in the strict 

 sense constituents of the latter, and which collectively have been called the 

 angioblast. While the mesamceboid cells are probably identical with the 

 primitive lymphocytes, the idea that they constitute a set of specific rudi- 

 ments of entodermal origin, from which both blood cells and endothelium 

 arise, has not been generally accepted. The view, however, is not discord- 

 ant with the monophyletic concept of the origin of blood cells. 



FIG. 247. Portion of a blood vessel from the yolk sac of a rabbit embryo, showing various 



stages in the formation of erythrocytes. Maximow. 

 0, megaloblasts; a', megaloblast in mitosis; b, normoblasts; b', normoblast in mitosis; c, erythro- 



blasts; d, erythrocyte, not yet discoid; en, endothelium; /, primitive lymphocytes; 



k, normoblast recently divided; n, shrunken erythroblasts (?); n', extruded nucleus. 



The primitive lymphocytes (of Maximow), constituting the parent stem 

 from which all the blood cells arise according to the monophyletic theory, 

 specialize at first in two general directions. In one direction the specializa- 

 tion leads toward the erythrocytes, or red blood corpuscles, and in the other 

 toward the leucocyte, or white blood cell, series including the myelocytes. 

 In the former case the lymphocytes become modified in that the cytoplasm 

 becomes less basophilic and acquires a trace of haemoglobin; the nuclei become 



