UMBILICAL CORD OF THE PIG 11 



/ may be regarded as a later stage in the intracellular eryth- 

 rocytogenic function of h, in which two of the nuclei and their 

 enveloping cytoplasm have differentiated into erythroplastids 

 (ep). Cells h, f, b, and e of figure 15 and e of figure 6, when 

 considered in common, demonstrate that erythroplastids in these 

 vasofactive cells do not arise as such out of the cytoplasm, but 

 under the direct influence of a nucleus of a bi- or multinucleated 

 hemoblast (memogenic giant-cell'). The absence of free eryth- 

 rocytes and erythroplastids in the regions from which these cells 

 are taken contravenes any suggestion that the intracellular red 

 blood-corpuscles should be interpreted in terms of phagocytosis. 

 Figures 12 and 14 supply similar evidence. The erythro- 

 plastid of figure 12 may appear to have arisen directly from the 

 cytoplasm of this vasofactive cell. But figure 14 shows an 

 essentially similar cell at a slightly earlier stage of differenti- 

 ation, in which the chief nucleus has liberated a small bud. 

 About this bud an erythrocyte and a lumen may be conceived 

 to originate in the manner shown in e of figure 15, and so lead 

 to a condition like that of figure 12. In other words, the cells 

 of figures 12 and 14 are essentially multinucleated hemoblasts. 

 In this sense multinucleated hemoblasts, hemogenic giant-cells, 

 and blood-islands ('angioblasts') are fundamentally and poten- 

 tially alike ; that is, they are essentially multiple ery throblasts 

 enveloped by a layer of potential endothelium. 



DISCUSSION 



From the foregoing description it will be clear that the con- 

 nective tissue of the full-term umbilical cord of the pig is exten- 

 sively vascularized and that it is actually for the most part still 

 in the condition of young mesenchyma or embryonal connective 

 tissue. The conditions are essentially similar to those described 

 for the body-stalk of very young human embryos. The question 

 arises whether the connective tissue of this cord is in the prim- 

 itive mesenchymal condition because it is vascularized or 

 whether it is vascularized because the connective tissue is in the 

 condition of undifferentiated mesenchyma. Since the blood- 

 vessels have apparently arisen to a considerable extent in situ, 



