50 VASCULOGENESIS IN THE CAT 



connected with plates and strands of mesenchyme, which are the 

 presumptive anlages of the aortic arches and the adjacent mesen- 

 chyme. Amidst this tissue are a few blood islands. 



The precise source of the cells which form the endothelium of 

 the aorta is somewhat difficult to determine. The clear cut con- 

 tour of the somites until the formation of the sclerotomes makes 

 it difficult to believe that cells are here given off to the mesen- 

 chyme. Mollier has held that its material is derived from the 

 intermediate cell masses, and has shown sections in which they 

 are connected to the aorta by sohd processes, similar to those 

 which elsewhere give rise to mesenchyme and endothelium. 



Such connections also are found in the cat. At the stage of 

 eight somites, and more clearly in that of twelve (figs. 12 and 13) 

 the aorta becomes joined to a more lateral point of the \dsceral 

 mesoderm. From the convexity which this layer forms below 

 the coelomic angle a ridge of cells projects ventro-mesad. This 

 is separated in many places from the mesoderm sensu strictu 

 by a narrow cleft, and thereupon comes into apposition with the 

 aorta(figs. 29 to 33) ; in places the cells of this ridge become endo- 

 thelial and contain a lumen which eventually communicates with 

 that of the aorta. It is posssible that the connection with the 

 intermediate cell mass described by Mollier may be of a similar 

 nature, and that in both cases we are dealing with an increment 

 to already formed endothelium. The anlages of the aorta both 

 in front and behind the somites are freely connected with the over- 

 lying paraxial mesoderm and it is probable that they are directly 

 derived from it prior to its segmentation. Later when the scler- 

 otomes are formed the aorta enters into intimate relation to 

 their loosely arranged cells (figs. 29 to 33). The addition of 

 elements from this source to the aortic endothelium has been de- 

 scribed by Mollier, ^^ and his results are abundantly confirmed 

 by conditions in the cat. Some of these cell masses acquire a 

 lumen before addition to the aorta. In other sections the vessel 

 loses its well defined outline mesad and its wall is completed by 

 typical mesenchyme, with the interstices of which its lumen com- 



" Loe. cit., ]). 12()7. 



