59 



of these are said to arise extra-vascularly. They are not descendants 

 but sister cells of the primary erythroblasts. They are not found any- 

 where else according to Schridde. They are held not to be swept 

 there by the blood current. And they cannot arise from mesenchyme 

 cells since such are absent from the liver. There is histologic evidence 

 for their origin from the cells of the vessel wall. 



It appears then that the 13 mm embryo and its umbilical vesicle 

 here under consideration are at a crucial stage, according to Schridde's 

 hypothesis. Evidence ought to appear here in favor or in contradiction 

 of Schridde's findings, and in this respect this single specimen is 

 extremely important. In the first place, I believe that I have found 

 in the blood islands of the umbilical vesicle earlier developmental 

 stages than those described by Schridde as hemoglobin-containing 

 primary erythroblasts. One sees here the central cells of an island 

 passing through various stages in the formation of normoblasts. The 

 peripheral cells become arranged into an endothelial wall. The earliest 

 stage of the central cells correspond to Maximow's "lymphocyte". 

 It is represented by a polymorphous, amoeboid cell with large basophile 

 nucleus and a hemoglobin-free, light-staining granular protoplasm. 

 In subsequent stages of differentiation and proliferation the cells 

 become smaller, and, due to their crowded condition, modified 

 spherical and polyhedral in form. Plasma appears and the cells 

 float free in the fessel. Concomitantly with these changes, endo- 

 thelial cells may pass through similar stages and enter the vessels 

 as normoblasts. 



Furthermore, Schridde differentiates between the primary and 

 secondary erythroblasts — the former arise in the yolk-sac, the latter 

 in the liver — both of which types exist together, on grounds of size 

 of cell and nucleus. In both respects the secondary erythroblasts are 

 said to be smaller. Moreover, the secondary are described as arising 

 extra vascularly and the primary erythroblasts intravascularly. Also, 

 the primary erythroblasts are said to be two to three times or even 

 four times as large as the definitive erythrocyte. 



In ray specimen no difference appears in the size of the cells 

 and nuclei, nor in depth of staining reaction, nor in the condition of 

 the cytoplasm, between the cells of the yolk sac and those of the liver. 

 Similar types of similar sizes exist everywhere. Nor are the cells 

 ordinarily more than twice as large as adult erythroblasts. The 

 following table compiled from measurements of a large number of cells 

 illustrates the above statements : 



