51 



nmnicates. Finally the mesal angle of the aorta is often occupied 

 by masses of adherent bood cells, as though the sclerotome had 

 given rise not only to endothelium but blood cells as well. 



To summarize : the formation of endothelium in the splanchno- 

 pleure is precisely analogous to that in the somatopleure. Mesen- 

 chyme is formed in situ and secondarily in large part is transform- 

 ed into endothelium. There are few solid strands of mesenchyme 

 ('angioblast') and these are not primary but arise from the con- 

 crescence of scattered plates of mesenchyme. They are rarely 

 transverse, as though they had grown in, but conform to the 

 lines of future vessels. More important than their frequent 

 interruptions as evidence of in situ origin, are their innumerable 

 connections with the compact mesoderm, every degree of emer- 

 gence from which, both by migration of single energids and by 

 delamination, is conspicuous in every embryo examined. 



ADDENDUM 



Since the foregoing paper has been hi proof a communication made 

 by Bremer at the thirtieth session of the A.A.A. in Philadelphia and 

 published in abstract (Proc. Am. Ass. Anat., Anat. Rec., vol. 8, no. 2) 

 makes a brief consideration of the results of this -investigator desir- 

 able. In his first paper (Am. Jour. Anat., vol. 13, 1912) Bremer 

 describes the development of the intraembryonic blood vessels in the 

 rabbit as resulting from the ingrowth of a continuous network of 

 "angioblast cords" derived from the extraembryonic plexus of the 

 yolk-sac. Although dealing in this paper with the development as 

 found in rabbit embryos he examined other species, as chick, pig, 

 sheep, etc., and felt "satisfied that in all essential points the story 

 of the development of these primary vessels in other vertebrates will be 



found similar " The evidence on which these conclusions rest 



is briefly the continuity of the aortic and cardiac angiocysts with 

 the extraembryonic vessels of the splanchnopleure by solid cords of 

 cells, which are interpreted as angioblast. The essential data are 

 given succinctly and the principle of his interpretation made clear 

 in the' description and figure of the rabbit embryo of 5 segments. "Fig. 

 1. a reconstruction of the angioblast cords of one side of a rabbit em- 

 bryo of five segments, shows these cords, streaming in from the net- 

 work over the yolk-sac They have grown from left to 



right of the figure, occasionally anastomosing until near the median 

 line, \vhich lies at the left of the figure. The shape of the meshes of this 

 net indicates it seems to me the direction of this growth and the rapidity 

 with which it has occurred." I question both the angioblastic nature 



