GENERAL CONSIDERATIONS 61 



photographically the concrescence of separate vascular anlagen 

 in the area pellucida of the chick's blastoderm in vitro. 



If we compare the development of the intra-embryonic blood- 

 vascular channels, as determined by observation and experiment, 

 with that of the plexus which arises on the yolk-sac, we find, in 

 the genesis of their endothelium from mesenchyme, and in their 

 formation by a concresence of independent anlagen, that the intra- 

 and extra-embryonic blood-vascular channels follow exactly 

 the same genetic plan. 



If one attempted to follow the development of these intra- 

 or extra-embryonic blood-vascular channels by means of in- 

 jections, it is evident that this method would reveal only the 

 extent to which a continuous system of injectible lumina had 

 been established at the time the injections were made. It 

 would fail completely to reveal the facts which have been 

 definitely determined by experiment, that the injectible lumina 

 had been previously formed by a concrescence of independent 

 and uninjectible vascular spaces, and that the endothelium which 

 forms the walls of these lumina had been formed in situ, not from 

 a preexisting endothelium, but from mesenchymal cells. 



Since we now know that the intra-embryonic blood vessels, 

 like those in the yolk-sac, are formed by a concrescence of in- 

 dependent anlagen, and that their endothelium is formed in 

 situ from mesenchymal cells, the question naturally confronts 

 us as to the method by means of which these independent an- 

 lagen become connected with one another to form a system of 

 vessels with continuous lumina, that extend throughout the 

 body of the embryo. 



There appear to be only three possible methods by means of 

 which such connections could take place: (1) Either by means of 

 a proliferation or migration of the cells of which the original 

 independent anlagen are composed; (2) by a further local in 

 situ differentiation into endothelium of the embryonic cells which 

 intervene between the independent anlagen; or (3) by a combi- 

 nation of these two methods. 



We all recognize the fact that endothelium, like other tissues 

 of the body, is capable of growth after it has once been formed. 



