462 JOHN LEWIS BREMER 



From the differences in the conclusions reached by two of these 

 authors it seems certain that more work should be done along these 

 lines before a consensus of opinion can be expected. I wish to 

 point out a few possibilities which should, I think, be considered 

 in any such future work. 



As was shown in the figures of my reconstructions of rabbit 

 embryos, the vascular net has an irregular mesial border, certain 

 strands lying further toward the midline than the position of the 

 future aorta. Though in the younger embryos the extension of 

 these strands across the median line of the embryo proper to form 

 a net on the opposite side is rendered impossible by the close 

 approximation of the medullary groove, notochord, and ento- 

 derm, yet long before the stage figured in many instances cited 

 by these authors the mesodenn has grown across the median line, 

 and might afford a pathway for endothelial sprouts from side to 

 side. Another and earlier pathway is at the posterior end of the 

 embryo, behind the primitive streak, where the mesoderm very 

 early extends across the median line. The angioblast cords, by 

 which connections from recognizable blood-vessels to apparently 

 isolated angiocysts can be traced (if we accept, for the moment, and 

 for the purpose of argument, the extension theory) are delicate 

 strands, easily overlooked, and moreover may last only a few 

 hours, if the mechanical conditions are not favorable to their 

 continued development into vessels (cf. figs. 1 and 3, loc. cit.). 

 It is not to be expected, therefore, that anything short of a very 

 complete series of such operated embryos, fixed at progressively 

 longer intervals of incubation after operation, can settle whether 

 or not there is any extension from the opposite side. 



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 perhaps help to explain the frequently described 

 endothelial spaces unconnected with any injectable vessels. The 

 angioblast cords are apparently solid cords of cells, connected end 

 to end or in small groups, running between the processes of the 

 surrounding mesenchymal cells, when these are present, often 

 touching them, without however actually fusing with them. The 

 diameter of the cords is never as small as that of the mesenchymal 



