VASCULOGENESIS IN THE CAT 43 



tinguished from the other mesenchyme cells by their transverse 

 position, in addition to their slightly more abundant protoplasm 

 and slightly paler stain. It is possible that the latter two char- 

 acteristics may be due to the imbibition of fluid. The anlages 

 of the umbilical line are disposed along the lateral margin of the 

 coelom. A mesal line of sunilar anlages is forming above the 

 nephrotomes, while a few scattered elements have an intermediate 

 position. In all these situations the process is discontinuous, 

 the masses of vasofactive cells as well as the cavities are separated 

 by appreciable intervals in which ordinary mesenchyme is present. 

 They can be made into a continuum only by the inclusion of this 

 tissue. The result would be a sheet of mesenchyme studded with 

 scattered endothelial anlages as in the model of the tweh'e so- 

 mite embryo (figs. 12, 13). 



A second embryo of twelve somites (Columbia Collection No. 

 540) is shown in figure 14 for comparison with the embryo just 

 described. The discontinuity of the anlages is again apparent. 

 In addition to those before mentioned in the umbilical line, in 

 the cardinal line (along the nephrotomes) and intermediate 

 between the two (somato-pleuric plexus) , an angiocyst has formed 

 just in front of the first somite. The transverse vasofactive 

 cells have increased in number and are effecting junctions with 

 one another and with the angiocysts. In a third embryo of 12 

 (Columbia Collection No. 536) somites, there were rather more 

 numerous vasofactive cells, rather less developed endothelium 

 but the general arrangement was very similar; the angiocyst in 

 front of the first somite was also present. 



The transverse position of the vasofactive cells requires ex- 

 planation. They stand out clearly against the other mesenchyme 

 cells which have their long axis in general radial, and are further 

 distinguished by their rather deep position, near or in the almost 

 cell-free interval between the bulk of the mesenchyme and the 

 compact layer of parietal mesoderm. The nephrotomes are now 

 resolving rapidly into mesenchjTXie, they are segmented, and are 

 separating from the coelomic mesoderm ; through the spaces thus 

 formed, which contain only a loose mesostroma, the hypecto- 

 dermal and hyperentodermal regions communicate. In the latter 



