CHAPTER IX 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



I. Primitive Blood- Vessels and Blood-Cells 



The blood-cells and primitive blood-vessels arise from a tissue termed by His 

 the angioblast. Its origin has been in doubt. According to Minot (in Keibel 

 and Mall, vol. 2), it arises in the wall of the yolk-sac from the endoderm and from 

 it endothelial sprouts grow into the body of the embryo. Another view as to its 

 origin, more recently championed by Maximow, Felix, Schulte and Bremer, is 

 that the angioblast arises from the mesoderm. The angioblast consists of a net- 

 work of solid cords of cells which appear first in the splanchnic mesoderm of the 

 chorion and yolk-sac. In human embryos with a medullary plate about 1 mm. 

 in length, Bremer (Anat. Record, vol. 8, p. 97, 1914) finds a network of angio- 

 blast in the chorion, chorionic villi and body-stalk. This chorionic angioblast 

 antedates in one case that developed in the yolk-sac and thus must develop in- 

 dependently from the splanchnic mesoderm. The solid cords of angioblast soon 

 hollow out, the peripheral cells forming the endothelium of the primitive vessels, 

 the inner cells 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 network. In the wall of the yolk-sac this network forms the area 

 vasculosa (Fig. 74), in which aggregations of blood-cells fdrm the blood-islands. 



THE PRIMITIVE BLOOD-CELLS OR MESAMCEBOIDS (MINOT) 

 These show large vesicular nuclei surrounded by a small amount of finely 

 granular cytoplasm (Fig. 243 a). They are without a cell membrane and are 

 assumed to be amoeboid. During embryonic life, the mesamceboid cells multi- 

 ply rapidly by mitosis and develop successively in the wall of the yolk-sac, in the 

 liver, in the lymphoid organs and in the red bone marrow. 



Minot (in Keibel and Mall) and many embryologists hold at the present time that the 

 blood-forming cells of the adult are derived directly from the mesamceboid cells of the embryo. 

 Maximow (Archiv. f. mikr. Anat., vols. 67 and 73, pp. 680-757, an( l 444~56i) maintains that 

 blood-forming cells may take their origin from the mesodermal cells in embryos and also from 

 mesenchymal cells of the adult connective tissue. From what is now known of the origin of 

 the primitive blood-cells Maximow's view seems to be the more plausible of the two. 



