86 FRANKLIN P. REAGAN 



the extremity of this projection from the entoderm there is a 

 vacuole in the tissue. To the right of this vacuole the tissue is 

 solid; from this region there can be traced anteriorly through 

 many sections a longitudinally coursing column of cells. An- 

 teriorly this column contains intermittent vacuoles. The splanch- 

 nic mesoderm concresces around this column of cells and (figs. 3 

 and 43) the apices of the folds produced fuse mesial to the col- 

 umn of cells which has here (fig. 3) taken on the undoubted 

 characteristics of early endothelium. In figure 6 the endothe- 

 lium is considerably flattened, the myocardial tube has severed 

 connection with the splanchnic mesoderm and now lies inside 

 the pericardial cavity, appearing free in section. Anterior to 

 this (figs. 9 and 10) both endocardium and myocardium end 

 abruptly. The gross structure of this formation has already 

 been considered. 



Summarizing the conditions just described, we have here a 

 column of cells, the base of which rests on the entoderm and is 

 being actively contributed to by that epithelium. Whether the 

 entire column has increased by the activity of this one growing- 

 point can not be stated with certainty. It has probably increased 

 also by its own growth. Anteriorly this column of cells possesses 

 the characteristics of endothelium and is surrounded by a myo- 

 cardium; both end abruptly. Whether the portion which is 

 truly endothelial has come from entoderm, the reader may judge 

 for himself. I strongly favor the interpretation that we have here 

 a true entodermal origin of prevascular tissue. 



Of the numerous cases observed in which pre-endothelial 

 mesoderm appeared to be given off by entoderm, I shall describe 

 one more. This is the case of meroplast Type II, No. 148. 

 Figure 44 will serve to locate the region more highly magnified 

 in figures 45, 46, 47 and 48. As we have already seen the right 

 incision was very close to the embryonic axis, leaving little meso- 

 dermal tissue on that side. On the left side the incision was 

 farther laterad, so that the tissue which remained could undergo 

 concrescence. The plane of this section is through the potential 

 heart-region. In close contact with the entoderm is an endo- 

 thelial tube with a dorsally projecting solid process. Let us now 



