72 FRANKLIN P. REAGAN 



to continue a dorsal up-folding. In some cases the parietal 

 layers remain in a practically horizontal position appearing as 

 rather inconspicuous appendages of the visceral lamellae which 

 have evidently continued their growth. This size-difference is 

 shown somewhat in figure 35. 



Let us now consider the behavior of the locally formed endo- 

 thelial tubes during this process. Since in all cases considered 

 the operations were performed at a time before the embryonic 

 fundament was vascularized (as was in most cases actually 

 verified by an examination of the blastoderm themselves) the 

 endothelial tubes in the trunk-region are local formations. Since 

 these endothelial tubes lie in the region of normal heart-forma- 

 tion, their behavior is of interest. 



In figure 23, morphologically ventral and internal to the 

 line of incision on each side will be seen in section an endo- 

 thelial tube. That on the right is the larger, as has already 

 been noted. The adjacent splanchnopleure has invaginated so 

 as to conform to the size and curvature of the surface of the 

 endothelial tube on each side. The conditions remind one of 

 mammalian heart-formation prior to the ventral approach of 

 the bilateral anlagen. It is impossible to say just how far the 

 process would have continued if the tissue had been left to further 

 incubation. 



The trunk-meroplast Type II, No. 116 is of interest in that it 

 portrays the process of concrescence as it may be followed pro- 

 gressively from the posterior to the anterior region. Fortu- 

 nately the incision on the left side (figs. 25 and 26) was nearer 

 to the median line in the posterior region than it was in the 

 anterior region; thus in the posterior region the tissue remain- 

 ing intact on the left side is much smaller in amount than on the 

 right. As the line of incision on the left side gradually diverged 

 from the median axis anteriorly, the tissue on the two sides 

 became equal by the time the incisions reached as far anteriorly 

 as the tubular head-region. On the left sides of figures 25 

 and 26 it is seen that the formation of endothelium has been 

 inhibited in the posterior region. Farther anteriorly, however, 

 endothelium has formed on this side so that the median heart 



