OKIGIN OF VASCULAR TISSUES 81 



potentially endothelial, mesothelial, dermal, or connective tissue. 

 The fonnation of endothelium on the teleost yolk-sac by wan- 

 dering mesenchyme cells a great distance from mesothelium 

 (77, 50, 66, 53) does not speak for a mesothelial origin of endo- 

 thelium. 



2. The origin of the prevascular mesoderm (mesenchyme) 



Thus far, the present account has not been concerned with 

 the origin of the prevascular mesoderm, except with evidence 

 against the view that this mesoderm is an ingrowth from the 

 yolk-sac vessels. It has been noted that Hahn and Graper 

 each believed himself to have ascertained which of the primitive 

 layers is responsible for this vascular tissue. It has generally 

 been considered by individual authors that one germ-layer, and 

 one alone, is capable of furnishing this tissue. Even those who 

 believe the direct precursor of endothelium to be mesenchyme 

 would probably derive that mesenchyme from one of the germ 

 layers and from no other. ' Considerations of the supposedly 

 high specificity of endothelium have given rise to this view; a 

 sponsor of such a view would probably maintain that endo- 

 thelium is such a highly specific substance that its origin from 

 more than one germ-layer is unthinkable. On the other hand 

 it would probably be maintained that mesenchyme is such a 

 heterogeneous tissue that any degree of equipotentiality in 

 mesenchyme of different sources would be equally unthinkable. 

 Considerations such as these probably explain the fact that 

 different germ-layers as producers of the vascular tissue have 

 generally been regarded as mutually exclusive. Yet when one 

 takes into account the ability with which each of these views 

 has been defended it seems possible that each may contain an 

 element of truth. The immediate section of the present dis- 

 cussion is devoted to a possible reconciliation of the two views 

 — namely the mesodermal and the entodermal origin of the 

 prevascular tissue. 



a. Endothelium from visceral mesoderm, Let us first consider 

 the posterior axial region. An experimental study as above out- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 21, NO. 1 



