CHAPTER Vll. 



THE VASCULAll SYSTEM. 



In various regions of early embryos, groups of mesenchyme cells 

 begin the development of the vascular system. The central cells 

 of such an area become rounded and separated by a fluid inter- 

 cellular plasma. The peripheral cells of these regions unite to form 

 an endothelial tube enclosing the free primitive blood cells and the 

 plasma. The thin walls about these spaces are interconnected with 

 others, so that gradually a network of endothelial-walled tubes 

 forms the first capillary system. Some of the early ca])illaries 

 develop into arteries, others into veins, and a tubular part is 

 later differentiated into the heart. In the development of an artery 

 and a vein, not only is an enlargement of the tube brought about, 

 but there is also an addition of fibroelastic connective tissue and 

 smooth muscle organized in sheaths about the endothelial lining. 



THE CAPILLARIES. 



These narrow, delicate, endothelial-walled tubes form a vast net- 

 work in the connective tissue throughout the body. The diameter 

 varies from a minimum of slightly less than the diameter of an 



Fig. 64. — Diagram of part of a capillary network, showing endothelial cells 



erythrocyte to a diameter several times this. (Figs. 64 and 65.) 

 In cross-sections of very small capillaries only one or two endothelial 

 cells form the wall, l)ut in larger tubes a number of cells are present. 

 The boimdaries of the endothelial cells show as irregular black lines 

 after silver nitrate treatment. The cells are elongated in the direc- 



