THE EARLIEST BLOOD-VESSELS IN MAN 



JOHN LEWIS BREMER 



Harvard Medical School, Department of Anatomy 



ELEVEN FIGURES 



In a former publication^ it was shown that in a rabbit embryo of 

 five segments the intra-embryonic vascular arrangement consists 

 of a net of solid endothelial cords, with occasional expanded 

 portions, called angiocysts, ill which a lumen is present. The net 

 occupies the area, just dorsal to the entoderm, between the lateral 

 border of the embryo proper, where it connects with the yolk-sac 

 net, and the site of the future aorta, on either side of the neural 

 groove. At the mesial border of the net numerous longitudinal 

 anastomoses indicate the position of the future aorta ; these anas- 

 tomoses are not complete, so that the aorta is in three sections, 

 not connected into a longer vessel. In the embryo then figured 

 the shape of the meshes of the net, the fact that each section of the 

 aortic net is separately connected with the lateral net, and the 

 further fact that in the posterior part of the embryo the mesial 

 border of the net lies progressively farther and farther from the 

 median line — all these facts seem to me to point to the invasion 

 of the embryo by this endothelial net, the actual growth of the 

 endothelium of the yolk-sac vessels into this new territory. Both 

 before the publication of this article and more recently, other 

 investigators have cast some doubt on this conclusion bj' experi- 

 ments on growing chick embryos, in which the heart, the aorta, 

 and other vessels are found to develop on both sides of the embryo, 

 even after the destruction of the yolk-sac vessels of one side. 

 Since the present paper may help to throw some light on the points 

 at issue, I shall defer a discussion till later. 



' Bremer, .1. L. Am. Jour. Anal . Vol. 13, p. 3, U)V2. 



447 



