UMBILICAL CORD OF THE PIG 9 



tiated into a hemoblast, and subsequently differentiated an 

 erythroplastid intracellularly. 



The mesenchymal cells of this primitive 'mucous' connective 

 tissue may apparently undergo any one of several types of dif- 

 ferentiation: 1) They may separate from the mesenchymal 

 syncytium, round up and differentiate into potential hemoblasts, 

 which may lie freely among the undifferentiated mesenchymal 

 cells, but apparently never in this condition directly metamor- 

 phose into erythroplastids ; but grouped into blood-islands, about 

 which the adjacent mesenchyme differentiates into endothelium, 

 they develop into erythroblasts (fig. 7). 2) They may become 

 bi- or multinucleated and, as hemogenic giant-cells, differentiate 

 erythrocytes intracellularly (figs. 6, e, and 15, /). 3) A mesen- 

 chymal cell may acquire a lumen and join with other cells to 

 form an initial capillary, incidentally differentiating also 

 erythroplastids intracellularly (fig. 12). Erythroplastids may 

 originate intracellularly also in young endothelial cells (fig. 10). 

 Hemoblasts can, therefore, apparently differentiate into erythro- 

 cytes only when inclosed by endothelium; or in the multinucleated 

 condition, hemoblasts can differentiate intracellular erythrocytes. 

 The latter phenomenon is essentially like that where a hemo- 

 blast is inclosed by endothelium. 



Figures 6 and 7 illustrate an earlier and later stage, respectively, 

 in the differentiation of a blood-island. In figure 6 the blood- 

 island is still largely a syncytium. However, endothelium can 

 be seen forming on its surface, and several of its cells are taking 

 on erythroblast ('megaloblast') features (a, b, and d). The cell 

 d has developed a large vacuole at one pole; this vacuole may 

 form part of the subsequent lumen. Cell e has differentiated 

 an erythrocyte. Several intercellular spaces have appeared in 

 the syncytium; these are the forerunner of the subsequent lumen, 

 to which certain intracellular spaces may also contribute. In 

 figure 7 the endothelium and the lumen are developed further, 

 and the hemoblasts are mostly in the erythroblast stage and 

 are generally separated from each other by cell membranes. 



It seems desirable at this point to indicate the chief differ- 

 ences, nuclear and cytoplasmic, between the young mesenchymal 



