70 FRANKLIN P. REAGAN 



tral wall of the meroplast is really the dorsal wall of the pharynx 

 The anterior axial tissue is very degenerate in appearance. At 

 about the beginning of the mid-brain region there is an extra- 

 ordinarily abrupt change in the tissue to a very perfectly normal 

 and healthy condition. This experiment yielded the most 

 satisfactory result which I have been able to obtain, due to some 

 favorable condition which I have never been able to duplicate. 

 Two very well developed aortae are present. Figure 18 is a 

 section through the blastoderm of this same embryo. Its plane 

 pasf es through the hind-brain region where the first axial vessels 

 should be expected. It shows the freedom of the area pellucida 

 from endothelium. 



d. Splanchnopleural concrescence and the mechanics of heart- 

 formation. So far we have considered the conditions in the 

 head region where a portion, at least, of the body axis was 

 tubular at the time of operation. It is evident that the region 

 of the body which becomes tubular by the folding and constric- 

 tion of flat layers must have some interesting experimental possi- 

 bilities. In the trunk-region the nearness of the longitudinal 

 incisions to the median line profoundly affects the reaction which 

 the tissue will give. If the incisions be relatively near that line 

 the three germ-layers retain their horizontal position, or at 

 least show no tendency towards concrescence (fig. 31; the right 

 sides of figs. 24 and 44). If the incisions be made slightly more 

 lateral (fig. 39) there is a rather abortive attempt at concres- 

 cence to produce an alimentary tract. One side will exhibit 

 this tendency, even though the incision on the opposite side 

 was so close to the body-axis that there could be no concrescence 

 (figs. 24, 25, 35, and 44). This shows conclusively that the 

 process is not one of mutual attraction of the opposite sides. 

 Figure 23 shows in a most striking manner the close relation 

 existing between the nearness of incision to the body axis and 

 the degree of concrescence. In this instance the incisions are 

 unequally distant from the median line, and the side containing 

 the greater amount of tissue underwent the greater amount of 

 concrescence. There is almost mathematical exactness in this 

 relation of the amount of intact tissue to the angle of concres- 



