ORIGIN OF VASCULAR TISSUES 83 



cord of cells which was observed in section to connect with the 

 dorsal aorta; the latter is not shown in this figure. The vascu- 

 lar structures in this figure are not connected with those in figure 

 38; the two are very far apart in the series. The figures de- 

 scribed will doubtless suffice to show the principle involved. 



b. Endothelium from parietal mesoderm. It is possible also by 

 experiment to obtain endothelium in a location where it could 

 not have developed from entoderm. An example of such a lo- 

 cation is the dorsal surface of the parietal mesoderm. These 

 observations confirm in every way the previous observations of 

 Schulte (63). Here, singly or in groups, cells become dorsally 

 proliferated; before or after losing connection with the meso- 

 derm (figs. 37 and 38) they may acquire cavities, thus forming 

 cysts in eveiy way comparable to those of the splanchnopleure; 

 these cysts may be connected by solid cords of cells or by cyto- 

 plasmic filaments. Here we have pre-endothelial tissue com- 

 pletely separated from entoderm by two layers of coelomic 

 mesoderm and by coelomic cavity. There is no observable 

 mesial communication between visceral and parietal vessels, and 

 certainly none by way of the yolk-sac. Obviously we are justi- 

 fied in regarding mesoderm as a possible source of endothelium. 



It will be noted in figure 37 that the ectoderm and entoderm 

 have failed to fuse. In instances similar to this I have found 

 that a tube of splanchnic endothelium may grow out laterally 

 until it projects beyond the line of longitudinal incision. This 

 is not only an example of the ability of endothelium to grow, 

 but it is also an illustration of the fact that the direction of 

 growth is not necessarily mesial. In rare cases a vascular tube 

 'of the splanchnic variety may send a growth laterally and up- 

 wards around the fused mesodermal layers and form a parietal 

 vascular cavity. Actual parietal cavities most often exist, how- 

 ever, which could not have been formed by such a growth, hav- 

 ing no connection with the splanchnic vessels, the latter likewise 

 having no connection with yolk-sac vessels. 



c. Cardiac endothelium from entoderm? It is a familiar fact 

 that in many forms the endothelium of the heart appears from 

 the first to be closely applied to the pharnygeal wall, often seem- 



