UMBILICAL CORD OF THE PIG 15 



is only when such a cell becomes inclosed by endothelium that 

 it differentiates into a red blood-cell. Thus a group of such cells 

 may form and in consequence produce pressure upon the sur- 

 rounding mesenchyme, which then becomes transformed into 

 endothelium. Under these conditions the enveloped hemoblasts 

 become erythrocytes (figs. 6 and 7). Such endothelium is simply 

 an adaptive form of mesenchyme, as originally maintained by 

 Huntington 4 and by Schulte, 11 and it may subsequently return 

 to mesenchyme, remain as endothelium, or differentiate hemo- 

 blasts either intra- or extraluminally. Endothelium, accordingly, 

 develops originally by two different methods, both clearly repre- 

 sented in the specimen under consideration: 1) By adaptation 

 of mesenchyme about a blood island; 2) by vacuolization of 

 vasofactive mesenchymal cells ('angioblasts'). 



A point of special interest concerns the fact that the nucleated 

 periphery of a multinucleated hemoblast supplies the same 

 favorable conditions or factors for determining erythrocytogenic 

 differentation as an endothelial wall. This phenomenon becomes 

 intelligible when we consider that both endothelial cells and 

 hemoblasts are only slightly modified mesenchymal cells, and 

 that the latter, as vasofactive cells, may become hollowed out 

 to form the lumen of an original capillary or differentiate intra- 

 cellular erythrocytes. The central fact here pertains to the 

 obviously very minute difference between the environmental 

 conditions or stimuli that determine whether the same cell (the 

 potential hemoblast, 'vasoformative cell', or 'angioblast') shall 

 become an endothelial cell or an erythroblast. This suggests 

 that also the factors which determine whether the hemoblast 

 shall become a leucocyte or an erythrocyte, in accordance with 

 the monophyletic theory of blood-cell origin, are similarly rel- 

 atively subtle and of minute degree. Original confinement by 

 endothelial walls furnishes the stimulus which determines 

 erythrocytogenesis; extra vascular differentiation leads to granu- 

 lopoiesis. As shown by the recent experiments of Danchakoff, 1,2 

 the original poly valency of the hemoblast, however, is lost by 

 the erythroblast, and this degree of differentiation is irrever- 

 sible. An erythroblast freed from its endothelial confines and 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 26, NO. 1 



