192 



THE BLOOD VASCULAE SYSTEM 



abundance in the veins of the extremities, especially the lower, is prob- 

 ably to be interpreted in terms of a quadrupedal ancestral condition. 



The fact should be borne in mind that it is because of their relative 

 infrequency that valves are not often met with in those transections of 

 the smaller veins which are seen in nearly all microscopical preparations. 



THE DEVELOPMENT OF BLOOD-VESSELS 



The earliest anlage of the blood vascular system is a mesenchymalike 

 layer, the angioblast, which appears between the entoderm and mesoderm 

 at the distal pole of the yolk-sac (Fig. 210) at a very early stage of the 

 embryonic development (1 millimeter, Minot). In this layer appear accu- 

 mulations of rounded cells in the 

 form of anastomosing irregular 

 solid cords. The peripheral cells 

 become flattened to form an endo- 

 thelial tube; the central cells sepa- 

 rate and scatter in the vessels as 

 primordial blood-cells floating in a 

 plasma, probably a secretion prod- 

 uct of the cells. This network of 

 primitive blood-vessels grows to- 

 ward the embryo in the shape of 

 tubes and solid nucleated sprouts 

 (angioblast cords, Bremer) and, 

 converging to form two large ves- 

 sels, invades the tissue of the 

 embryo in the region of the de- 

 veloping heart as the vitelline 

 veins. Subsequently other ves- 

 sels, both arteries and veins, appear in the embryo. Such vessels are pre- 

 ceded by capillary plexuses, as demonstrated by Evans (Anat. Eec., 3, 9, 

 1909), in which the definitive vessels arise as paths in the original network 

 selected, enlarged, and modified under the influence of mechanical factors 

 incident to the flow of the main stream of the blood. There can be no 

 doubt that the original anlages of the blood-vessels arise by a confluence of 

 separate spaces (angiocysts), possibly always connected by angioblast 

 cords, and tubes formed in the angioblast; likewise there is no doubt that 

 the embryonic blood-vessels sprout as tubes and solid cords and thus grow 

 into adjacent regions (Fig. 211). But the features of vasculogenesis con- 

 cerning which there remain decided differences of opinion are (1) the 

 nature and origin of the angioblast, that is whether of mesodermal, ento- 



FIG. 210. A 13 MM. HUMAN EMBRYO. 



The chorionic vesicle is cut open, re- 

 vealing the embryo enveloped in the 

 amnion, and the yolk-sac (y.s.). X 1^. 



