82 FRANKLIN P. REAGAN 



lined, if concentrated on the posterior axial region, would proba- 

 bly lead to the conclusion (as did the work of Hahn) that meso- 

 derm is the one source of the vascular tissue. Hahn figured a 

 number of cases in which the splanchnic mesoderm was giving 

 rise to ventrally directed cells or cell-groups in different stages 

 up to their transformation into endothelium. Such cases were 

 more numerous and striking in the posterior region; he believed 

 that the same process was involved in the formation of endo- 

 cardium. Certainly in the posterior region there can be little 

 doubt that such mesodermal proliferation takes place. Here 

 the dorsal surface of the entodermal layer is generally perfectly 

 smooth, seldom having its basement membrane interrupted. 

 In the posterior axial region the incipient endothelial tubes or 

 plugs are in practically all cases from their first appearance more 

 closely associated with the mesoderm than with the entoderm. 

 . It has already been noted that a dorsal aorta formed locally in 

 meroplast Type II, No. 49. Figure 31 shows a section of the 

 posterior axis of this embryo. On the left side the plane of sec- 

 tion passes through one of the independent anlagen of the aorta. 

 Anteriorly and posteriorly in the aortic line it is found in num- 

 erous places that the mesoderm is proliferating ventrally di- 

 rected groups of cells, some of which are solid (figs. 32 and 33). 

 The plane of section of figure 32 is much anterior to that of fig- 

 ure 31 ; it is a higher magnification showing the tissue lying left 

 of the notochord, including the aortic line on that side. The 

 plug of tissue projecting ventrally from the mesoderm is not in 

 continuity with other similar proliferations. Figure 33 shows 

 conditions somewhat comparable to these. This one experi- 

 ment has yielded all gradations from these solid tissue-plugs to 

 tubular endothelium. Figure 37 shows a series of such down- 

 growths from the splanchnic mesoderm. The plane of section of 

 figure 38 is far anterior to that of figure 37 and the magnification 

 of the latter is greater. Fi'om the splanchnic mesoderm in fig- 

 ure 37 one sees pre vascular proliferations in all stages up to the 

 formation of what might formerly have been regarded as angio- 

 blastic cords, angiocysts or 'sprouts' from these. On the left 

 side of figure 29 there runs mesially from the small cyst a solid 



