ORIGIN OF VASCULAR TISSUES 59 



of chick embryos from the extraembryonic blastoderm. The 

 conditions which they obtained were very similar to the results 

 of Hahn. Their work contributes two distinct advances over 

 that of Hahn. They reconstructed the endothelial tubes ob- 

 tamed and still more important, their method of separation of 

 the axial portion of the body from the blastoderm was by incision 

 rather than by the crude and uncontrollable method of cauteri- 

 zation or burning. The amount of injury along the hne of 

 separation is certainly much smaller in the method introduced 

 by Miller and McWhorter. Their work was submitted as proof 

 of the local origin of endothelium. Prior to, and following its 

 publication the objections urged against this work were inclu- 

 sive of the following: (1) the incisions may not have been made 

 sufficiently close to the embryonic axis; (2) they may not have 

 been made sufficiently early; (3) endothelium may have grown 

 in from the opposite side or from the ends. Prior to my own 

 work (52) the only published comment on the interpretation of 

 local origin in such experimental cases, besides a short discus- 

 sion by Schulte is that of Bremer (4, p. 462) who in discussing 

 the work of Graper, Hahn, Miller and McWhorter, states: 



From the differences in the conclusions reached by two of these 

 authors (Graper and Hahn) 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 consid- 

 ered in any such future work. 



As shown in my recoiistructions 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 young 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 entoderm, yet long before the stage figured 

 in many instances cited by these authors the mesoderm 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 argu- 

 ment, the extension theory) are delicate strands, easily overlooked, and 



