ORIGIN OF VASCULAR TISSUES 45 



claims that mesenchyme can transform into endothelium, and 

 that endothelium can transform into mesenchyme. 



The theory of local mesenchymatous origin of endothehum 

 dates back to the work of Goette (15) and Reichert (56). Within 

 more recent times, Riickert and MolUer (60) and their students 

 have chiefly constituted the European School who maintain 

 that endothehum develops in situ. Maximow (31), Bonnet (2), 

 von Felix (13) and others have also supported this view. In 

 this country, Huntington (19, 20) and McClure (32-37) and 

 their students have stood practically alone as sponsors of the 

 local oiigin theory. 



It is of interest to note some of the methods which have been 

 employed by the advocates of these conflicting views. Investi- 

 gations supporting the angioblast theory have sometimes been 

 based on studies of serial sections, frequently on relatively late 

 growth stages in the living tissues of amphibian larvae and other 

 tissues, and especially within recent times, on injections. Cer- 

 tain objections to the results obtained by these methods have 

 been raised by the opponents of the angioblast school. Espe- 

 cially Huntington, McClure, and Schulte have urged that intra- 

 vital study of late capillary plexuses, as has so often been made, 

 [Platner (47), Remak (57), Kolliker (25), Langer (26), Clark (7), 

 etc.] is merely a study of endothelial proliferation and not a 

 study of its genesis. To quote McClure (36), pp. 61-2) : 



We all recognize the fact that endothelium, like other tissues of the 

 body, is capable of growth after it has once been formed. In no other 

 manner could we account for the increase in size which blood-vessels 



undergo in the embryo It is also possible for anastomoses 



to be formed between different vessels by means of growth or sprouting 

 of their endothehal walls so that, in some cases, an increase in their 



extent through growth may actually take place From 



whatever standpoint it may be considered, the growth of endothelium 

 is of secondary significance as regards the problem at hand, since the 

 main question at issue does not concern the possibility that endothelium 

 may or may not grow, but rather how the endotheliimi is formed that 

 does the growing. 



The injection method has also been criticized as a method 

 capable of giving only one result — a negative result with refer- 



