CHAPTER IX 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



The Primitive Blood Vessels and Blood Cells 



Both the blood cells and the primitive blood vessels arise from a tissue termed 

 by His the angioblast. Its origin has long been in doubt. According to Minot 

 (in Keibel and Mall, vol. 2), Ruckert and others, the angioblast arises in the wall 

 of the yolk sac from the entoderm. 



Recent investigations by Maximow, Felix, Schulte, and Bremer point, however, to a 

 mesodermal origin. In the body stalli of very young human embryos, Bremer (Am. Jour. 

 Anat., vol. 16, 1914) has shown the direct origin of angioblast from splanchnic mesothelium. 

 Moreover since this angioblast may antedate that of the yolk sac an entodermal origin is 

 excluded. A further view, favored by Hertwig (Lehrb. d. Entw., 1915), derives the blood 

 cells from entoderm, the vascular endothelium from mesoderm. 



The angioblast consists initially of isolated solid cords and masses of cells 

 which appear first in the splanchnic mesoderm of the body stalk and yolk sac. 

 The soHd cords of angioblast soon hollow out, the peripheral cells forming the 

 endothelium of the primitive vessels, the inner cells, bathed by a clear fluid, persist- 

 ing as the primitive blood cells or mesamoeboids of Minot. By the union of the 

 isolated vascular spaces, the cellular network is soon converted into a vascular 

 plexus which completely covers the human yolk sac. In the wall of the yolk sac 

 this network is termed the area vasculosa, and here aggregations of blood cells 

 form the blood islands (Figs. 33 and 79). 



Haemopoiesis 



Two sharply contrasted views are held as to the mode of origin {hcemopoiesis) 

 of the various adult blood elements. According to the monophyletic theory, a 

 common stem- or mother cell such as the mesamceboid gives rise to all types of 

 blood elements, both red and white. The polyphyletic theory, on the contrary, 

 asserts that the erythroplastids and the several kinds of white cells are derived 

 from two or more distinct mother cells. The evidence seems to favor the mono- 

 phyletic view, although Stockard (Amer. Jour. Anat., vol. 18, 1915) has recently 

 advanced strong experimental evidence in support of the opposing theory. 



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