THE DEVELOPMENT OF BLOOD-VESSELS 



193 



dermal or of dual origin ; (2) the manner of origin of the primary vascular 

 stems in the embryo, whether by invasion .through growth from the extra- 

 embryonic primitive vascular area, or by a process in the body meseiichyrna 

 similar to that through which the primitive vessels arose in the yolk-sac 

 (umbilical vesicle). The evidence seems to favor the mesodermal origin 

 of the angioblast. The advocates of vasculogenesis by invasion (Evans, 

 Minot, Bremer and others) regard the original angioblast, very early differ- 

 entiated from mesenchyma, 

 as the sole future source of 

 endothelium, to which is 

 ascribed a strict specificity 

 throughout development. The 

 advocates of the in situ 

 method of origin ( Maximo w, 

 Huntington, Schultze, Miller 

 and others), on the contrary, 

 conceive early vasculogenesis 

 as a process of progressive 

 fusion of tissue spaces and 

 mesenchymal cells involving 

 a continued differentiation of 

 endothelium from mesenchy- 

 ma. 



The total evidence seems 

 to favor the view that in 

 earliest stages blood-vessels 

 may arise in the mesenchyma 

 of the embryo and that these 



primitive stems may be added to by discrete anlages all of which may 

 fuse to form the vascular net out of which develop the future main 

 vessels. 



The vessels of later embryonic and fetal stages probably arise solely 

 as sprouts from these earlier stems. 



The chief point of uncertainty concerns the point in time when vascu- 

 logenesis passes from a process including sprouting and fusion of separate 

 anlages, to one where extension is exclusively by terminal growth. Both 

 arteries and veins have a like origin in capillary plexuses. 



The final anastomosing sprouts of endothelium represent the defini- 

 tive capillaries. The development of the definitive wall of arteries and 

 veins involves the formation of extra-endothelial layers of muscular and 

 connective tissue elements from the surrounding mesenchyme, and their 

 association into the several tunics of the various subdivisions of these 

 vessels. 



FIG. 211. 'VASOFORMATIVE' CELLS FROM THE 

 MESENTERY OF A RABBIT SEVEN DAYS OLD. 



g.s., red blood cells; n, nucleus of the vascular 

 endothelium; p, points of growth, at which ex- 

 tension occurs. Highly magnified. (After Ran- 

 vier.) 



