ORIGIN OF VASCULAR TISSUES 41 



recent years the origin of the endotheUum which Hnes the haemal 

 and lymphatic vessels has perhaps been the central problem of 

 anatomical investigation. Over this question there has been an 

 extended controversy; even now it would be premature to say 

 that all the problems confronting us are completely settled. 



The principal points of difficulty with which one must reckon 

 in consideration of this peculiarity of endothelial tissue are 

 raised by these questions: Does physical continuity of endo- 

 thelial cells necessarily imply genetic continuity? Conversely, 

 is it possible that cells out of continuity with already formed 

 endothelium can ever become continuous with and a part of 

 that endothelium? It has been observed that in most classes 

 of vertebrates the site of endothelial formation is the yolk-sac. 

 Later, endothelium is found progressively nearer the embryonic 

 body and finally inside the embryonic body. As has been so 

 many times stated, there are two views concerning the nature 

 of this endothelium which is found associated with the intra- 

 embryonic tissues. One view is that the earliest vascular tissue 

 grows toward the embryo, and on reaching the embryo's body 

 permeates the intraembryonic tissues in a centrifugal manner, 

 forming the entire lining of the haemal and lymphatic systems. 

 This view has been designated as the angioblast theory of Wil- 

 helm His. Let us consider a few quotations from certain of the 

 supporters of this theory; in this manner we can study first- 

 hand the conceptions of these observers concerning the impli- 

 cations of this so-called ingrowth or extension theory. 



I. THE ANGIOBLAST THEORY 



Minot's conception of the angioblast (Keibel and Mall, Human 

 Embryology, vol. II, pp. 498-9) is as follows: 



Comparative embryology teaches us that the first vessels appear on 

 the yolk-sac collectively and at one time. They form a unit anlage, 

 which we call briefly the angioblast according to the suggestion of 



His I am inclined to think that the angioblast .... 



forms itself not through the transformation of mesodermic cells aheady 

 present, but from the layer of yolk-cells, and from a reticulate group- 

 ing of themselves between the middle and lower germ-layers. The 

 angioblast probably maintains its complete independence throughout 



