4 VASCULOGENESIS IN THE CAT 



into our idea of specificity as applied to tissues than the commu- 

 nity of origin of their elements. It would seem, therefore* that 

 the advocates of specificity have charged themselves rather to 

 prove that endothelium, once formed, produces only endothelium 

 and never any other elements, as say blood or connective tissue, 

 than that it is peculiar in its origin. 



On the other hand, it has been held that endothelium is simply 

 a derivative of the mesenchyme from which it arises diffusely 

 by a multitude of separate anlages. This view does not under- 

 take a general interpretation of the mesoderm and mesenchyme 

 and the apparent variations of their mode of origin as determined 

 by the yolk-content of the ovum, but merely states that endo- 

 thelium arises out of elements which have once formed a part 

 of the general mesenchyme complex. A development of this 

 point of view is the adaptive theory of endothelium, which holds 

 that endothelium is a modification of mesenchyme, and by no 

 means a stable one at that, but owes its presence to hydrody- 

 namic forces, and when these cease to operate, endothelium dis- 

 appears as endothelium and reverts to mesenchyme, or gives rise 

 to products indistinguishable from those of mesenchyme. Endo- 

 thelium is taken to be a form which mesenchyme assumes in 

 certain positions which can not be retained with the loss of position 

 and is then analogous to the biaiometamorphoses of plants grown 

 under unnatural conditions, which are lost when they are returned 

 to their normal environment. 



It is conceivable that some supporters of the adaptive theory 

 of the origin of endothelium may not feel themselves obliged 

 to maintain a variety of products on the part of endothelium, 

 just as it is not to be required of those who hold the specific 

 doctrine that they should demonstrate an entodermal origin. 

 The former might then hold endothelium to be a derivative rather 

 than a modification of mesenchyme; it would satisfy them to 

 show the multitude of its separate anlages, their wide diffusion 

 through the mesenchyme, in the somatopleure as well as in the 

 splanchnopleure, within the body of the embryo as well as in 

 the yolk-sac, and their continued formation in the embryo dur- 

 ing an appreciable period of development, and along the major 



