HISTOGENESIS OF CIRCULATORY SYSTEM 731 



The angioblast theory rests upon the assumption that a special vascular 

 tissue, called the angioblast by Wilhelm His, develops in the area of the yolk 

 sac. This angioblast tissue, according to the angioblast theory, forms a vas- 

 cular rudiment within which the endothelium, or flattened epithelial cells 

 peculiar to blood capillaries, is developed. This endothelium produces the 

 primitive capillaries of the yolk area, and, further, it grows into the developing 

 embryo where it forms the endothelium of the entire intra-embryonic vascular 

 system. That 's, the angioblast in the yolk area provides the source from 

 which arises the endothelial lining of all the primitive blood vessels of the 

 embryo and also of all later endothelium of later blood vessels. The endo- 

 thelium of all blood vessels thus traces its ancestry back to the yolk-sac 

 angioblast. 



The local origin theory may be divided into two schools of thought. One 

 school espouses the idea that "mesenchyme may, in practically any region 

 of the body, transform into vascular tissue" (McClure, '21, p. 221). Accord- 

 ingly, primitive blood capillaries arise in loco from mesenchyme in various 

 parts of the embryo, and these local vessels sprout, grow, and become united 

 to form the continuous vascular system. The endothelium which forms the 

 walls of all capillaries and the lining tissue of all blood vessels of larger 

 dimensions forms directly from mesenchyme. Addition to this mesenchyme 

 may occur by proliferation from endothelium already formed or by the con- 

 version of mesenchyme as single cells or cellular aggregates (McClure, '21; 

 Reagan, '17). 



A second school which advocates the local origin of blood vessels differs 

 from the view described above principally by the assumption that, while the 

 endothelium of blood vessels appears to arise in loco from the mesenchyme, 

 it is not a generalized type of mesenchyme but rather a "slightly modified 

 mesenchymal cell" (Stockard, '15). Relative to this position, the following 

 quotation from Stockard, '15, p. 323, is given: 



The facts presented seem to indicate that vascular endothelium, erythrocytes and 

 leucocytes, although all arise from mesenchyme, are really polyphyletic in origin: 

 that is, each has a different mesenchymal aniage. To make the meaning absolutely 

 clear, I consider the origin of the liver and pancreas cells a parallel case. Both arise 

 from endoderm, but each is formed by a distinctly different endodermal aniage, 

 and if one of these two aniagen is destroyed, the other is powerless to replace its 

 product. 



3. Later Development of Blood Vessels 



While the capillary possessing a wall composed of thin, flattened endo- 

 thelial cells is the basic or fundamental condition of all blood vessels in the 

 body, it is only of transitory importance in the development of the arteries 

 and veins. For, in the formation of the arteries and the veins, the primitive 

 capillary enlarges and its endothelial wall is soon reinforced by the addition 

 of white and elastic connective tissue fibers and smooth muscle tissue. The 



