EARLIEST BLOOD-VESSELS IN MAN 459 



Briefly summarized, my observations point to ingrowths of the 

 mesothelial layer covering the yolk-sac and body-stalk as the 

 anlages of the blood-vessel endothelium and, to a lesser extent, of 

 the blood-corpuscles. The anlages, though limited to certain 

 areas commensurate with the extent of the mesothelial layer, are 

 multiple and form a net by the growth and coalescence of the 

 separate units. A further extension, accompanied by the incor- 

 poration of other units, effects the union of the body-stalk net 

 with that on the yolk-sac. On the other hand, the net in the 

 chorion and chorionic villi seems to me to be formed by direct 

 extension from the body-stalk, without the addition of new com- 

 ponents, and to be the result of a centrifugal growth of endothelial 

 sprouts, in the form of angioblast cords, with here and there 

 expanded angiocysts, which advance through the mesenchyma. 

 This would correspond with the fact that no surely isolated endo- 

 thelial cords have been found in the chorion, that in the Herzog 

 embryo the net extends only part way around the chorion and is 

 centered at the base of the body-stalk, and that there is in the 

 embryos studied no mesothelium on the inner surface of the 

 chorion proper, and therefore no possibility of new anlages there. 

 The chorionic mesothelium is developed later, but only after the 

 vessels are well established. 



That the angioblast cords and angiocysts are direct sprouts 

 from the endothelium and not mesenchymal cells metamorphosed 

 in situ and added to the growing tip is shown (aside from the 

 recent work of Clark" and others on endothelial growth) by the 

 presence of the clear-cut extra-intimal space (figs. 10 and 11) and by 

 the fact that there are no protoplasmic connections between endo- 

 thelium and mesenchyma, even in the most distal vessels. The 

 irregularities occasionally found in the outlines of the endothelium 

 seem to me to point to its contraction or perhaps to amoeboid 

 movements incident to the formation of new sprouts. That the 

 extra-intimal space may be an artefact, that the so-called cords 

 may be in reality collapsed tubes, is, to my mind, immaterial, and 

 awaits proof by injection methods; the sharp boundary between 



11 Clark, E. R. Am. Journ. Anat. Vol. 13, p. 351, 1912. 



