ORIGIN OF VASCULAR TISSUES 71 



cence. Incidentally it might be mentioned here, as will be con- 

 sidered later, that the size of the endothelial tubes obtained 

 (fig. 23) also bears a close relation to the amount of tissue left 

 intact. It will be noted (figs. 23, 25, 30, 38, 39, and 44) that 

 there is a tendency for ectoderm, both layers of mesoderm, and 

 entoderm to remain united at the line of incision. When this 

 happens, as it ordinarily does, the parietal mesoderm and the 

 ectoderm share in this ventrally directed concrescence. Thus 

 in some cases the tubular meroplast produced by the fusion of 

 these ventrally directed complexes may be entirely surrounded 

 by ectoderm (fig. 28) or partly so, as in figure 27, in which a part 

 of the ventral 'body wall' is composed of entoderm. In this 

 latter connection it is profitable to consider again the condi- 

 tions in figure 17 where, as we have already seen, the entire 

 ventral 'body wall' is entodermal. On the right side of this 

 figure it will be noted that the incision was relatively close to 

 the body axis. Whatever attempt there may have been made 

 on this side at a ventral concrescence was annulled by the 

 remarkable dorso-ventral expansion of the tissues, which even 

 caused the entoderm to bend upwards. On the left of the figure 

 it will be seen that the incision was located somewhat more 

 laterally — obviously more lateral than the most lateral extent 

 of the potential dorsal wall of the pharynx. The apex of the 

 projecting fold on the left side represents the point of fusion of 

 ectoderm and entoderm. This fold is somewhat ventr^dly 

 directed, so that here we have a feeble attempt at a ventral 

 concrescence. It would undoubtedly be possible to get a com- 

 pletely graded series of trunk-meroplasts from those whose entire 

 ventral wall is entodermal, to those entirely surrounded by 

 ectoderm. 



It does not always happen that parietal and visceral meso- 

 derm remain united, as one sees in figure 35; but in all cases 

 this parietal complex tends to be somewhat ventrally directed. 

 In no instance have the parietal layers folded dorsally to simu- 

 late amnion-formation. It may be that the mechanical condi- 

 tions of the operation cause the mesodermal layers to be united 

 for a time, and that when they part, the upper complex is unable 



