14 H. E. JORDAN 



The matter may be summed up with figures 15, a, b, and e, 

 and 15, h and /. A connective-tissue cell becomes transformed 

 into a hemoblast (erythroblast) with vasofactive capacity. This 

 cell may become bi- or multinucleated. One or several of the 

 nuclei with their enveloping cytoplasm may differentiate into 

 erythrocytes. Meanwhile a lumen appears within the cell, and 

 one or two of the original nuclei may persist as the nuclei of the 

 peripheral cytoplasm of the differentiating cell, which now forms 

 the endothelial wall of the initial capillary. In later stages in 

 the yolk-sac and in the red bone-marrow generally, the peripheral 

 'endothelial' layer of the original 'vasofactive' cell disappears, 

 thus freeing the intracellularly differentiated erythrocytes into 

 the confining blood spaces. The mesenchymal cell thus appears 

 endowed with divers hemogenic "potentialities: it may become 

 an endothelial cell or a hemoblast (erythroblast) . The endothe- 

 lial cell may secondarily differentiate into a hemoblast. These 

 hemoblasts may differentiate into erythroblasts or, as multinu- 

 cleated cells, they may differentiate both intracellular erythro- 

 cytes and a potential endothelial cell. These facts demonstrate 

 the very close relation between mesenchyme, endothelium, and 

 hemoblasts. 



Sabin 9 records a similar vacuolization of mesenchymal 'angio- 

 blasts' in the living blastoderm of the two-day chick embryo 

 grown in Locke's solution, by which the blood-vessel lumen 

 forms. But these observations do not justify her conclusion 

 that they prove "that the lumen of a blood-vessel is intracellular" 

 (p. 200). The data supplied by the umbilical cord of the pig 

 show that the definitive lumen of the blood-vessel derived from 

 a blood-island is of both inter- and intracellular origin. 



This brings us to the matter of the factors which determine 

 whether the mesenchymal cell shall become an endothelial cell 

 or a hemoblast, and relates this investigation to the discussion 

 regarding theories of hemogenesis, that is, whether blood de- 

 velopment proceeds according to the monophyletic or the poly- 

 phyletic mode. This much seems certain regarding this tissue: 

 single hemoblasts, freed from the mesenchyme and wandering 

 within its meshwork, d*o not differentiate into erythrocytes. It 



