8 H. E. JORDAN 



themselves in line with the capillaries and ultimately become 

 incorporated as part of the capillary network. In the full-term 

 condition of the cord, blood-vessels arise in a manner identical 

 with their primary origin in the original body-stalk, and become 

 secondarily connected with the preexisting vascular net. The 

 connective tissue of the full-term cord maintains the primitive 

 vasculogenic mode by which the primitive blood-vessels were 

 formed. The full-term cord is relatively much more extensively 

 vascularized, and it consists of connective tissue largely of a 

 less differentiated type than the cord of the 21-mm. fetus. Since 

 the larger blood-vessels extend to the distal end of the cord, it 

 may be inferred that they supply also the proximal pole of the 

 allantois. 



VASCULOGENESIS AND HEMOPOIESIS 



In the description of the mode of vasculogenesis illustrated 

 in this cord, we may begin most conveniently with the stage 

 represented by the cell of figure 8. This cell has the general 

 characteristics of an irregular mesenchymal cell. Three vacuoles 

 can be seen to the right of the nucleus. A later stage may be 

 represented by the upper cell of figure 9. Here the cell is binucle- 

 ated, and the originally smaller discrete vacuoles have presumably 

 coalesced to form a single large vacuole, the precursor of the 

 initial capillary lumen. The appearance of the lumen has ef- 

 fected a modification of one of the nuclei so that it begins to 

 assume endothelial features. In figure 10 is shown a still later 

 phase of vasculogenesis. Here, moreover, the more typical endo- 

 thelial 'cell' has taken on hemoblast features and has differentiated 

 an erythroplastid (ep.) intracellularly. Figure 11 may be con- 

 ceived to represent a transection of the cell of figure 9 or 10. 

 Cells like those of figures 8, 9, 10, and 11 are very numerous in 

 the more peripheral regions of the cross-section. Figure 12 

 illustrates a binucleated cell with essentially mesenchymal fea- 

 tures, which has differentiated an erythroplastid and a lumei^ 

 centrally. The cell shown in figure 13 has the nuclear and cyto- 

 plasmic characteristics more of a young hemoblast. This cell 

 represents a mesenchymal cell which has rounded up and differen- 



