31 



matopoietic function is of considerable importance, however, even if 

 only cooperative, is certain, for, as Hubrecht (21) points out, "the 

 liver cannot be said as yet to furnish a sufficient number of blood- 

 corpuscles for assisting in the metabolic processes of the primate 

 embryo". 



In the umbilical vesicle of the 13 mm embryo the erythroblasts 

 (normoblasts) can be traced to their origin from mesenchyme cells. 

 In the body of the embryo (blood vessels, liver and heart) all the 

 types are found that occur in the vesicle. The preponderating type,^ 

 however, is the normoblast. No non-nucleated or enucleating cells could 

 anywhere be found. Nor can any leucocytes, barring the so-called 

 "lymphocytes" (Maximow), blood-mother-cells, be demonstrated. These 

 lymphocytes were found only in the blood islands and the circulating 

 blood — not as the wandering cells in the embryonic mesenchyme except 

 possibly in the liver. Occasional megaloblasts are also present in 

 the circulatory blood. Mitoses are frequent, especially among the 

 megaloblasts (Fig. 12 c^), both in the embryo and in the yolk-sac, though 

 rather more numerous in the former. Occasional binucleate normo- 

 blasts can be found (Fig. 12 c^). 



The absence of evidence of hematopoiesis (other than proliferation 

 of the cells already in the blood channels) in the embryo, and the 

 evidence of such (blood islands) in the umbilical vesicle, points to the 

 latter as of very real significance in this connection. That blood-cell 

 formation should continue in the umbilical vesicle of an embryo of 

 this age, and in the light of the above facts gratuitously, would be 

 strange indeed. Manifestly it is impossible, however, to assert defin- 

 itely that all the corpuscles of the embryo had origin in the umbilical 

 vesicle. But the evidence points strongly towards the umbilical origin 

 of the very great majority of them up to this stage. 



It remains to describe the blood islands. Since only a single 

 stain was employed, the chief marks of differentiation are morpho- 

 logic. Depth and lightness of stain, and the granular or homogeneous 

 character of the cytoplasm, however, characterize consistently certain 

 types of cells and to this extent give confirmatory information. More- 

 over, the several cell-types agree in structural characters very closely 

 with those differentiated on the basis of delicate staining reactions 



mesoderm of these several structures possesses great vascular potency 

 irrespective of position. But the evidence is perhaps as yet to meagre 

 to "warrant the general conclusion that in man the earliest hematopoietic 

 centers are normally extra-omphaloidean. 



