ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 289 



ever — 1) that in certain cells with more than one normoblast no 

 haemoblast nucleus remains (fig. 26) ; 2) that giant cells of the 

 yolk-sac are simply modified haemoblasts whose cytoplasm 

 undergoes a chemical alteration, as indicated bj^ staining re- 

 actions, similar to that of haemoblasts in becoming erythro- 

 blasts; 3) that no multinucleated giant cells could be found in 

 process of fragmentation into mononucleated cells; 4) that the 

 cytoplasmic relationship between the two cells is frequently very 

 intimate; 5) that such intracellular normoblasts are occasionally 

 in mitosis, an unexpected phenomenon in ingested degenerating 

 cells; and the possibility 6) that the cells interpreted as phago- 

 cytic endotheha may indeed be cells differentiating normo- 

 blasts intracellularly while still attached to the blood vessel 

 wall — all indicate that the structure in question is one repre- 

 senting actual intracellular differentiation of normoblasts within 

 a giant cell. This matter will be further discussed below. 



In the yolk-sac of the 25 mm. pig embryo the blood vessels 

 are relatively much larger. No blood-islands occur. The blood 

 cells are predominantly of the normoblast type; there are also 

 some erythroblasts and a few haemoblasts. Giant cells are 

 apparently lacking; and the endothelium of the blood channels 

 is apparently no longer capable of haemoblast formation. 



IV. DISCUSSION 

 a. Function of yolk-sac 



1) Digestive. The yolk-sac entoderm is of course continuous 

 with the epithelial lining of the gut through the yolk-stalk. 

 Originally similarly undifferentiated, the yolk-sac entoderm 

 already at the 5 mm. stage has far outstripped the gut entoderm 

 in differentiation. Even at the 10 mm. stage the cells lining 

 the gut are relatively little differentiated. The chief mark of 

 functional activit}'- on the part of the yolk-sac entodermal cells 

 is the presence of a generous amount of basal filaments. Such 

 are lacking in the gut entoderm of this stage. These filaments 

 resemble very closely mitochondria; they may be long or short, 

 straight or variously curved, delicate or coarse, apparentlj'- 



