96 CARNEGIE INSTITUTION OF WASHINGTON. 



sels of the chorion are quite small. They form a rich plexus of open 

 channels occupying the thickness of the vesicle wall. Except in the 

 immediate neighborhood of the body stalk, they contain no formed 

 blood elements. The vessels of the villi are abundant, to some extent 

 forming channels, but otherwise consisting of solid cords. In many 

 cases they exist as detached strands. In the yolk sac the vascular 

 rudiments consist of blood islands and vessels in various stages of for- 

 mation, irregularly distributed over the vesicle, being more numerous 

 in the region of the fundus. They reach their most advanced stage of 

 development posteriorly, near the attachment of the body stalk, where 

 they form the vitelline plexus, providing a partial anastomosis between 

 the vessels of the embryo and those of the body stalk. Apparently 

 the posterior aortic antedates the omphalomesenteric connection. 



As for the blood-vessels of the embryo proper, in the heart region the 

 first pair of aortic arches exist as definite endothelium-lined channels 

 from which minute sprouts extend forward as the internal carotid 

 arteries, while longer channels extend caudalward as the dorsal aortse. 

 The latter are discontinuous, slender vessels, either solid or hollow, 

 which extend along the margin of the neural plate. The heart is en- 

 tirely plexiform, and it is of peculiar interest to note that it shows no 

 evidence of bilaterality aside from its relations to the dorsal aortse 

 and the interrupted omphalomesenteric plexus. 



A consideration of the vascular features presented by this embryo 

 completely confirms the idea of the ability of the mesoderm to give rise 

 to vascular endothelium, at least at certain stages and in man. In 

 the body stalk and chorion the process of vessel formation by meso- 

 dermal accessions is perhaps already superseded by the direct growth 

 and extension of preexisting vessels. In the yolk sac and the embryo 

 both processes were still going on and it is evident that the vessels of 

 the embryonic body were being largely formed in loco instead of by 

 invasion from without. 



It can be seen that a vascular system so incomplete and disconnected 

 can not at this time play a functional role of any great importance to 

 its possessor. The needs of the embryo as regards a circulating or 

 diffusible medium must be met by the relatively great surface which 

 its component parts present to the surrounding fluid, as in the yolk 

 sac and coelom. There can be no doubt that important nutritive 

 material finds its way to the young embryo through the coelom or, 

 secondarily, through the yolk sac. We do not yet know whether the 

 fluid within the human umbilical vesicle differs materially from that 

 outside. One may conceive, however, that its lining endoderm plays 

 some role in this regard. Dr. Ingalls points out that, even after the 

 establishment of a complete, closed circulation, there still remain in 

 the embryo wide intercellular and often avascular spaces which must 

 function in the exchange of various fluids. Gradually, however, this 



