66 FRANKLIN P. REAGAN 



formed without the intervention of a parenchymatous stage; 

 that such isolated vesicles coalesce to form the tubes; that such 

 tubes are enlarged by the addition of single mesenchyme cells, 

 or by the addition of solid or hollow groups of cells. There 

 appear certain cases, however, exhibiting a parenchymatous 

 arrangement of cells which, from their staining reaction, are to 

 be interpreted as true endothelial cells. 



In figure 14 is shown a cross (perhaps frontally inclined) 

 section of a head-meroplast. On the right the incision was very 

 close to the brain tube. The dorsal mesenchyme has arranged 

 itself into the form of an epithelium. The incision included the 

 pharynx, causing it to gap open and be displaced by tension of 

 the opposite side. The cut edges of ectoderm and entoderm 

 end blindly. Lateral to the vcntrallj^ directed U-shaped por- 

 tion of the pharynx an endothelial tube is seen on each side. 

 Between these two tubes is a solid ventral connection. The 

 conditions here resemble heart-formation to a certain extent. 

 The coelomic walls have met mesially, but their place of meet- 

 ing is ventral to the pharynx; the latter has not been constricted 

 for the reason that it was already tubular at the time at which 

 the operation was performed, whereas in heart-formation the 

 transformation of flat entoderm into tubular pharynx is an 

 essential process. A more accurate figure of the ventral region 

 of a neighboring section is shown in figure 21, in which many 

 structures are figured less diagrammatically. 



Conditions somewhat similar to these are shown in figure 15. 

 The longitudinal incision, as seen on the left of the figure, was 

 not far from the median plane, and here again the mesenchyme 

 has formed an epithelium which merges so gradually into ecto- 

 derm that it is impossible to determine where that layer begins. 

 It will be noted that on the left side of this embryo no endo- 

 thelium has formed. When the incision is made so close to the 

 median plane, such conditions often result; it seems to stimu- 

 late a vigorous growth of mesenchyme which becomes so dense 

 that the formation of endothelium is impossible. Such close 

 incisions are not necessary for the prevention of vascular in- 

 growth. 



