42 FRANKLIN P. REAGAN 



life. In other words, it is probable that the endothelium of the blood- 

 vessels (and of the ha'nph-vessels) and the blood cells at every age are 

 direct descendants of tlie primitive angioblast. 



The following is from Evans (Keibel and Mall, vol. II, p. 571) : 



In embryos of the higher vertebrates the first cells which can be 

 identified as standing in any relation to the vascular system appear in 

 the form of localized thickenings of the extra-embryonic mesoderm 

 lying next the entoderm of the yolk-sac. These constitute the so- 

 called vascular anlagen and typically undergo a gradual differentiation 

 into two or more definite cell-types, blood cells on the one hand, and 



endothelial cells on the other It is important, then, to 



distinguish vessels which have arisen through sprouting of other vessels 

 in contrast to vessels whose endothelimn has been contributed directly 

 from the neighboring mesoderm. Even on the yolk-sac these latter 

 vessels are not numerous, for they occur only at the site of the so-called 

 anlagen, and the main mass of the vitelline capillary plexus arises 

 from the extension and frequent anastomosis of these primary vessels. 

 . . . . In birds it has been possible to establish beyond all doubt 

 that most of the aorta descendens is formed from the medial margin 

 of the vitelline capillary plexus. 



Bremer (3) agrees with the angioblastic view. He believes, 

 however, that the injection method is inadequate for mapping 

 out the extent of the vascular tissue, even in case of the caudal 

 aorta, as the establishment of its lumen is not a continuous proc- 

 ess, so that isolated cavities actually exist, but the cells of their 

 walls are in genetic continuity. He states (p. 114) that ''it 

 was the observance of these isolated spaces which later fuse to 

 form large vessels, that lead to the often repeated statements 

 of Riickert and Mollier and others that the dorsal aortae arise 

 in situ from cells of the mesoderm." 



In 1914 (4) his conception of the angioblast is much modified 

 (p. 462) : 



I think it well at this point to define more accurately what I mean 

 by angioblast cords, especially since I believe that their recognition 

 may help to explain the frequently described endothelial spaces uncon- 

 nected with any injectible vessel. The angioblast cords are appar- 

 ently solid cords of cells connected end to end or in small groups, run- 

 ning between the processes of the sm^rounding mesenchyme cells, when 

 these are present, often touching them, without actually fusing with 

 them. The diameter of the cords is never as small as that of the 



