ERYTHKOPOIESIS IN YOLK-SAC OF PIG EMBRYO 297 



to be an essential one; they may represent simply atypical or 

 possibly ancestral phenomena. Erythrocytes commonly de- 

 velop from mononuclear haemoblasts; binucleated haemoblasts 

 apparently sometimes divide to form two haemoblasts (fig. 3, e) ; 

 multinucleated haemoblasts (polykaryocytes) do not break up 

 into mononuclear haemoblasts, but may produce erythrocytes 

 (normoblasts) intracellularly. 



V. SUMMARY 



1. In pig embryos of about 10 mm. length, the yolk-sac 

 attains its highest stage of progressive histologic differentiation. 

 This statement pertains both to the entoderm and to the angio- 

 blast. 



2. The entodemal cells are characterized chiefly by abundant 

 presecretion filaments, in which feature they agree with the 

 cells of the liver and mesonephroi. 



3. Angioblast arises from the mesenchyma. 



4. The mesothelium of the yolk-sac of pig embryos between 

 5 and 12 mm. does not produce haemoblasts. Nor is there any 

 satisfactory evidence that the mesothelium of the body stalk 

 and chorion function to this end. 



5. The mesenchyma may differentiate directly into endothe- 

 lium or into haemoblasts. 



6. Haemoblasts arise extensively at the 10 mm. stage from 

 the endothelium of the yolk-sac blood vessels. The endothelia 

 of the hepatic sinusoids and mesonephric glomeruli of this stage 

 also show extensive haemopoietic capacity. 



7. Giant cells, both mono- and polynuclear, are abundantly 

 present in the yolk-sac only at about the 10 mm. stage of develop- 

 ment. They may arise from endothelium or directly from haemo- 

 blasts. They are giant haemoblasts, and apparently function 

 as multiple erj^throblasts in which normoblasts differentiate 

 intracellularl}^ 



8. The several stages in haemopoiesis, represented successively 

 by haemoblasts, erythroblasts and normoblasts, with transition 

 stages, are abundantly present in the yolk-sac of embiyos from 

 5 to 15 mm. 



