12 H. E. JORDAN 



t he latter interpretation would seem to be the correct one; that 

 is, that this cord has maintained early embryonic conditions, 

 like that of its anlage, the body-stalk, and in consequence re- 

 tained its original capacity for vasculogenesis and erythrocyto- 

 genesis. The cause of this maintainance of early embryonic 

 vasculogenic and hemopoietic potentialities, especially singular 

 in connection with the advanced developmental condition of the 

 umbilical arteries and vein, and the several small areas of fully 

 differentiated mucous connective tissue, remains for the present 

 undertermined. It is most probably associated with the large 

 functional allantois, but the nature of this association is not 

 clear. The relatively highly developed character and healthy 

 condition of the covering ectoderm may be secondary to the 

 presence of the large number of capillaries in the subjacent 

 connective tissue. 



Though this study can throw no light on the cause of the 

 vascularized condition of the umbilical cord of the pig, the in- 

 tense hemopoietic activity of its connective tissue supplies valu- 

 able data with respect to the initial steps in vasculogenesis. This 

 is the chief point of value in this specimen. In this connection 

 interest centers upon the mesenchymal cell, which becomes 

 hollowed out to form an endothelial cell and at the same time 

 differentiates erythrocytes (figs. 6, 12, 13, and 15). This cell 

 combines the functions of an endothelioblast and an erythroblast. 

 The process appears to be quite similar to that first described by 

 Ranvier 8 (74) in the mesentery of the seven-day rabbit and in the 

 great omentum of the cat, and independently by Schaefer 10 (74) 

 in the subcutaneous tissue of the new-born rat, and subsequently 

 confirmed by other workers on other forms. Ranvier named the 

 cells concerned in the process 'vasoformative cells.' According 

 to these investigators, mature (non-nucleated) 'erythrocytes' of 

 greatly varying sizes are formed directly within the protoplasm 

 of connective-tissue cells (vasoformative cells) by a process in- 

 volving the coalescence of scattered granules of hemoglobin into 

 condensed globules, which then come to lie in vesicles within 

 the cells, the precursors of the capillary lumen. 



