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 but recent 

 investigations by Maximow, Felix, Schulte, and Bremer point to the 

 mesoderm. 



In the body stalk of very young human embryos, Bremer (1914) has shown the 

 direct origin of angioblast from splanchnic mesothelium. Moreover since this angio- 

 blast may antedate that of the yolk sac an entodermal origin is excluded. According 

 to Minot (1912), Riickert, and others, the angioblast arises in the wall of the yolk sac 

 from the entoderm. A further view, favored by Hertwig (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 solid cords of angioblast soon hollow out, the peripheral 

 cells forming the endothelium of the primitive vessels, the inner cells, bathed 

 by a clear fluid, persisting as the primitive blood cells, or mesamceboids 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 vaculosa, 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 

 (haemopoiesis) of the various adult blood elements. According to the 

 monophyletic theory, a common stem-, or mother cell, such as the mes- 

 amceboid, 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 total evidence seems to favor the monophyletic view, 

 yet there are able dissenters (Stockard, 1915). 



The Primitive Blood Cells or Mesamceboids. These show large, 

 vesicular nuclei surrounded by a small amount of finely granular cytoplasm 

 (Fig. 250 a). They are without a distinct cell membrane and are assumed 



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