62 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



In no other manner could we account for the increase in size 

 which blood vessels undergo in the embryo after they have 

 attained then* adult structure and form. It is also possible for 

 anastomoses to be formed between different blood vessels by 

 means of a growth or sprouting of their endothelial walls, so that, 

 in some cases, an increase in their extent, through growth, may 

 actually take place. It is therefore quite probable that growth 

 may play a considerable role in establishing a concrescence 

 between the independent endothelial-lined anlagen of the blood- 

 vascular system. From whatever standpoint it may be considered, 

 however, the growth of an endothelium is a feature of secondary 

 significance as regards the problem at hand, since the main ques- 

 tion at issue does not concern the possibility that endothelium may 

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

 does the growing. 



The distinction between the actual genesis of endothelium and 

 the growth it may undergo after it has once been formed is 

 naturally one that has been disregarded by those who maintain 

 that intra-embryonic vascular endothelium is not directly a local 

 product of mesenchymal cells. A special specificity has there- 

 fore been attributed by the supporters of the 'angioblast' theory 

 to the endothelium of the intra-embryonic vascular system, on 

 the ground that it takes its origin only from the yolk-sac angio- 

 blast. In accordance with this view, it is by means of one con- 

 tinuous and uninterrupted growth of a preexisting endothelium 

 (yolk-sac angioblast) throughout the body of the embryo, that 

 the endothelium of the blood-vascular and lymphatic systems 

 is formed. 



Since the 'angioblast' theory of His no longer holds, the ques- 

 tion of the specificity of tissues is involved in the vascular prob- 

 lem only to the same extent as is the case for any other tissue in 

 the body. Whether the mesenchymal cells of the embryo are 

 in an embryonic or undifferentiated state, and capable of further 

 differentiation into cells which form muscle, connective tissue, 

 endothelium, etc., is entirely beside the question; provided we 

 know that the product of these intra-embryonic mesenchymal 

 cells actually forms endothelium and that the latter is not de- 



