ARTERIES. 



tive cells, which form capillaries independently of already 

 existing vessels (Kanvier). These cells are associated with the 

 so-called intracellular development of red blood-corpuscles. In 

 their protoplasm are to be found well-formed red cells, as well 



FIG. 86. 







Buds from^ '. ^ Y^ ^^_^_- -Mood capillary 



capillaries* 



^ * 

 & 



^ * ! 



4 ^^" 



Kiidnthflial cell* 



Piece of the omentmn niajus of an eijjht day do.tr 's embryo, seen from the surface. X 180. 



as granules containing haemoglobin. Sig. Mayer considers 

 these cells, on the contrary, to be the result of degenerative 

 changes in capillaries, and claims that the haemoglobin-con- 

 taining granules are broken-down erythrocytes. 



(&) Arteries. 



The precapillary arteries show only a thin sheath of con- 

 nective tissue or a structureless elastic membrane outside the 

 endothelial tube. The accessory coats become thicker as they 

 approach the heart. Those of the smaller arteries begin as a 

 thin discontinuous layer of smooth muscle cells circularly 

 arranged around the sheath of connective tissue which covers 

 the endothelial tube. In somewhat thicker small arteries the 

 muscle cells form a definite circular coat (Figs. 87 and 88). In 

 the longitudinal section of such an artery the muscle-cell 



