58 



62 



in the umbilical vesicle and the embryo, though more abundantly in 

 the latter. Thus the earlier stages of hematopoiesis in man seem to 

 agree with the findings of Maximow for the rabbit and guinea-pig. 



The facts as far as they go 

 support the monophyletic theory 

 of blood-cell origin. 



Interpretation and 

 Discussion. 



The lymphocyte is the pri- 

 mitive blood cell descrised by 

 Maximow (31) as "eine ubiqui- 

 täre, indifferente, polymorphe, 



wandernde Mesenchymzelle". 

 ScHRiDDE (44) seems to recognize 

 no cell answering this description 

 in his scheme of blood-cell de- 

 velopment. In fact he expressly 

 states that "es keine primären 

 Wanderzellen gibt" as defined 

 by Saxer (40) and Maximow. 

 Maximow expresses the opinion 

 that ScHEiDDE "die wirklich 

 ersten Stadien der ersten Blut- 

 bildung nicht kennt". Schridde 

 divides the developmental pheno- 

 mena of the first human blood 

 cells into two stages: 1) Here 

 intravascular primary erythro- 

 blasts of the yolk sac and con- 

 necting-stalk appear. These are 

 said to arise out of the vessel- 

 wall- cells and to increase by 

 mitotic proliferation. 2) This 

 begins first in an embryo of 

 12,5 ram. The new cells are 

 limited absolutely to the liver. 

 They give origin at the same 

 instant to three distinct types 

 of cells: myeloblasts, secondary 

 erythroblasts and giant-cells. All 



^^H 



« 



R 



Fig. 12. Different types of blood cells 

 found in the wall of the umbilical vesicle of 

 the 13 mm embryo : 6^ „lymphocytes" (Maxi- 

 mow) ; c megaloblast ; d^ early generation of 

 normoblast ; (P later generation of normoblast ; 

 d} three young erythroblasts in a capillary; 

 e two older erythroblasts ; R multinucleate giant 

 <;ell; o^ dividing megaloblast (fi'om liver); c^ 

 binucleate megaloblast (from liver). The blood 

 cells found in the embiyo and its umbilical 

 vesicle appear identical. Erythrocytes, leuco- 

 cytes and small lymphocytes have not yet ap- 

 peared. X 1500. Reduced ^/^ iu reproduction. 



