VASCULOGENESIS IN THE CAT 5 



vascular lines. This done, they could allow endothelium to 

 manifest such specificity as it might acquire, as in the case of 

 other mesenchymal derivatives, cartilage for example, or cross- 

 striated muscle, which increase by accretion of new elements 

 as well as by the proliferation of those already formed, and yet 

 once formed, are specific to the degree that they probabably do 

 not give rise to other types of connective tissue. 



In thus comparing the angioblastic with the mesenchymal, the 

 specific with the adaptive views of endothelium, I have alluded 

 to certain differences of opinion regarding the supposed mode 

 of its wide dissemination through the connective tissue of the 

 body, which are fundamental and may now be contrasted. The 

 doctrine of specificity holds this process to be essentially one of 

 growth. The period of development of endothelium is amaz- 

 ingly short, and the site, for most entertainers of this view, is the 

 yolk-sac at some distance from the embryo. For others there is 

 an abstract possibility that certain parts of certain embryonic 

 vessels may develop in situ, but it is not unfair to say that they 

 consistently tend to minimize and curtail the developmental 

 period of endothelium. The important part of their doctrine, 

 the part which has engaged their chief attention, and is presented 

 with most conviction, is the growth of the endothelium by solid 

 sprouts from the ends of hollow injectible vessels. This is the 

 sole source of endothelium (the case of a few embryonic vessels 

 apart), and so far as I am aware, absolutely the only evidence 

 of its specificity. 



The adaptive theory finds support in the presence, through 

 a considerable period of ontogeny, of numerous separate sacs or 

 vesicles of endothelium, definitely localized in positions subse- 

 quently occupied by continuous vessels, and in the gradual en- 

 largement and ultimate fusion of these vesicles to form the ves- 

 sels. Further, it notes that the vesicles are preceded by minute 

 clefts or spaces in the mesenchyme bounded by ordinary mesen- 

 chyme cells, which subsequently flatten and eventually form a 

 continuous layer as the vesicles enlarge. This view thus extends 

 the period of development, and enlarges the area in which it is 

 active to practically the whole body of the embryo. It does not 



