450 JOHN LEWIS BREMER 



of the l)l()()d ill the Keibel and jNIall '' Human Embryology," where 

 it is spoken of as suggesting the area opaca and area pellucida of 

 lower forms. But, although the vascular net is much more 

 IDrominent near the distal pole of the yolk-sac, the net of slender 

 solid cords can be traced, in the area pellucida, nearty to the 

 embryonic shield. Before reaching the shield, the net in this 

 embryo comes to an end, though apparently unconnected cords, 

 resembling the angioblast cords, may be seen here and there 

 running for only short distances, extending even into the body- 

 stalk. 



In the chorion and chorionic villi of the same embryo there are 

 many of the ''irregular spaces" described by other authors; they 

 are cavities in the mesoderm, with distinct walls formed either by 

 flattened or spindle-shaped cells, or merely by the clean cut edge 

 of the loose mass of mesenchymal processes and fine fibrils in which 

 they lie. In single sections the larger cavities may appear abso- 

 lutely empty of any cellular content, but on reconstruction each 

 cavity is always found to contain a shred of tissue, apparently 

 floating in it. Not infrequently these shreds enclose small vacu- 

 oles, or may even open out into angiocysts, with a single layer of 

 cells forming the wall. They occupy only a small part of the 

 mesodermal cavities, as a rule, and seem to be loose in them, like a 

 thread run through a pipe ; often they lie close against the walls of 

 the cavities. The impression received from the study of these 

 shreds is that of an endothelium shrunken away from the rest of 

 the wall of a vessel. By making graphic reconstructions of these 

 inner shreds of tissue, I found that here too the pattern obtained 

 is that of a net, differing however from that on the yolk-sac by 

 extending in three planes, or being in several layers. The recon- 

 structions are not complete, in that many of the smaller branches 

 were not traced to their termination, since it was chiefly desired 

 to emphasize the net character of these cords. 



This characteristic arrangement of these cords and vesicles, 

 their general resemblance to other early endothelium, and other 

 facts to be brought out later convince me that we are here dealing 

 with the angioblast, or, perhaps better expressed, the endothelium 

 of future blood-\'essels. 



