VASCULOGENESIS IN THE CAT 41 



changes are at a maximum along the lateral margin of the coelom 

 where the parietal as well as the visceral layers are fairly resolv- 

 ing themselves into mesenchyme, to an extent that threatens 

 to interrupt theil- continuity — a conformation which character- 

 izes this region until the coelom is fully formed. In individual 

 sections there are many cells which appear free of the mesoderm 

 save for cytodesmata. The study of serial sections shows that 

 many of these are actuallj^ free (save for cytodesmata), but that 

 others are the tips of the processes of mesoderm just mentioned. 

 While the mesench\nne continues to be recruited by the migra- 

 tion of single cells, from this period on there occurs a more inten- 

 sive process, in which projections are formed and separate en 

 masse, and as these projections are inclined to the mesoderm at an 

 acute angle and often extend through several sections, this phase 

 of mesenchyme production may be described as delamination. 



In the embryo of four somites (Columbia Collection No. 409) 

 there is an interval between the mesenchyme of the lateral sheet, 

 and that in the head and in the region of the nephrotomes, occa- 

 sioned by the relative unproductiveness of the parietal layer of 

 the cardiac coelom, which is thin and compact. The two areas 

 become continuous opposite the last somite (fig. 6). 



Our next embryo is of eight somites (Columbia Collection No. 

 530) . The two areas of mesenchyme are now closer together and 

 the parietal layer of the cardiac coelom is active in the produc- 

 tion of this tissue (fig. 7). 



In both of these embryos we find the same intimate relation 

 between mesoderm and mesenchyme that has been described. 



There are evidences of active migration of single energids into 

 the mesostroma assisted by a coarser process of delamination. 

 In addition, there are evidences of the inception of endothelium. 

 Small clefts or cavities have appeared here and there amid the 

 cells of the mesenchjTiie, which are thus coming to enclose minute 

 vesicles. As these cells are in a syncj^tium, the question of the 

 locality of the cleft, whether inter or intra-cellular is of secondary 

 importance. The situation of these incipient vesicles (vaso- 

 factive cells) is indicated by the yellow color in figure 7. 



