ORIGIN OF VASCULAR TISSUES 87 



consider sections of this same general region more highly magni- 

 fied. Figure 45 is a section considerably anterior to the plane of 

 figure 44. Dorsally there is a projection from the entoderm; it 

 is continuous with certain other structures in this region whose 

 pre-endothelial nature can scarcely be doubted. While mitotic 

 figures are by no means wanting, they are not so numerous as 

 those in the case just considered. Figure 46 is a section through 

 the middle heart-region slightly anterior to the plane of figure 

 44. In fact the endothelial cavity of figure 46 can be traced dis- 

 continuously back to the endothelial cavity of figure 44. It will 

 be noted that the endothelial cavity in figure 46 is joined to the 

 entoderm by a solid cord of cells at the base of which an ento- 

 dermal cell is dividing. In the posterior heart-region (fig. 47), 

 interesting conditions obtain. Dorsally there is a plug of cells, 

 evidently being given off from the entoderm. There can be no 

 doubt that the basement-membrane is here interrupted. More 

 ventrally there is a similar proliferation. The latter is in con- 

 tact with, but not continuous with the visceral mesoderm. 

 Still more posteriorly the conditions are of great interest. Here 

 there appears to be a transition between a region where the en- 

 toderm seems to be the source of endothelium, and the more 

 posterior region where the mesoderm seems to be responsible for 

 all the endothelium. In figure 48 it will be noted that the base- 

 ment-membrane of the pharnygeal entoderm is interrupted and 

 that pre-endothelial cells lie in a matrix common with that of 

 the entoderm and also with that of the splanchnic mesoderm. 

 Destroy the entodermal connection with this pre-vascular com- 

 plex, and one would say that this complex had originated from 

 splanchnic mesoderm. Posterior to this region the basement 

 membrane of the entoderm is intact, and endothelial tissue has 

 no connection with any tissue other than visceral mesoderm. 



Figure 35 is a section just anterior to the heart-region of mero- 

 plast Type I, No. 34. Such a condition as this would probably 

 be interpreted as an instance of mesodermal origin of endothe- 

 lium by delamination. This section is more anterior than the 

 region represented by figures 45, 46, 47 and 48; it is the ventral 

 extremity of a left fold comparable to that of figure .44. 



