vasculoGenesis in the cat 37 



ation we have two independent subsidiaiy germ layers, mesen- 

 ch\Tiie and angioblast, which are absohitely distinct, though in- 

 separably united, and are in no sense phases or modifications of 

 one structural basis, is not an obvious conclusion and is insuper- 

 ably difficult in the view of the later permutability of endothelium 

 and mesenchjinal derivatives. 



The entodermal origin of some of these elements, the mesoder- 

 mal of others, the great proportional variation ®f the two com- 

 ponents in different ova, would seem to point to position in the 

 broad sense of the term, and not to derivation as the determin- 

 ing factor in their beha\-ior. The comprehensive and detailed 

 resume of Rtickert and Mollier, together with their individual 

 contributions seem to necessitate a view of this sort, for they 

 definitively dispose of an entodermal or a mesodermal origin of 

 vessels as a general principle of vaseulogenesis. 



Clearly therefore, though the causative factors of vaseulogene- 

 sis in the blastoderm are but imperfectly understood, yet in 

 the hght of present knowledge the source of the vasofactive cells 

 must be conceived as of secondary rather than of primary im- 

 portance. The doctrine of the angioblast can therefore find no 

 support in the facts of comparati\-e embryology in so far as it 

 hints an entodermic origin for endothelium and blood. But if 

 angioblast and mesenchyme are thus conterminous, arising 

 from the same sources, and possessing like potentialities, and have 

 further the most intimate topographical relations, it is difficult 

 to see how they have come to be separated conceptually by some 

 investigators, unless it be from their too intensive confinement 

 of attention to the splanchnopleure, were the excess and pre- 

 cocity of vaseulogenesis somewhat masks the presence of 

 mesenchjTiie. 



This is the phase of the problem which I have endeavored to 

 approach directly in the light of obser\'ations upon a series of 

 blastoderms of the domestic cat, ranging from the period of the 

 three-layered blastoderm, to the stage of fourteen somites. They 

 were fixed in Zenker's solution, embedded in paraffin, cut into 

 serial transverse sections 13^ ^ thick, and \'ariously stained — 

 the most satisfactory results on the whole being gained with 



