ERYTHROBLASTS IN TPIE PIG EMBRYO 169 



does not seem probable that they had been subjected to abnormal 

 pressure. In the case of figure 5, for example, the cell in question 

 was lying in a vessel having a diameter several times that of the 

 blood corpuscles, at a considerable distance from any capillaries, 

 and under conditions in which the surrounding blood cells were 

 not closely crowded. Fourth, the erythroblasts with these cyto- 

 plasmic constrictions are further characterized as having the 

 smaller more compact nuclei typical of the mature cell, whereas 

 similar constrictions were not observed in younger undifferen- 

 tiated erythroblasts with large nuclei. 



c. Size and form of the plastids 



Some attention has already been given to the variation in 

 size and form of the non-nucleated erythrocytes found normally in 

 the circulation of 25 to 35 mm. pig embryos (cf. p. 149 and figs. 

 4, 11, 26 to 31). From their size it seems self evident that the 

 larger plastids must have been derived from the large erythro- 

 blasts; (as for the question which some investigators have raised 

 to the effect that the first erythroblast do not give rise to non- 

 nucleated elements see discussion on pp. 176-177). At the 

 other extreme are hemoglobin-containing elements about the 

 size of blood platelets or even smaller, which it appears must have 

 arisen as subdivisions of a larger erythrocyte. In form both the 

 large and small plastids vary from cup-shape to biconcave discs in 

 both fixed and fresh preparations, with apparently a preponder- 

 ance in the direction of the cup or bell form (figs. 4, 29, 30). The 

 fact that even the smaller plastids may also present a disc or 

 cup shape is of interest since it appears highly probable that 

 they must have arisen as subdivisions of a larger corpuscle, 

 whether that process be one of fragmentation or of cytoplasmic 

 constriction of the character under consideration. That small 

 cytoplasmic subdivisions of erythrocytes may take on the typi- 

 cal form of the definitive plastid has been noted by Erlich ('85) 

 who finds tha't the so-called poikilocytes may assume "eine 

 deutliche Dellung" and concludes ''dass in dem abgeschniirten 

 Theil neben dem Hamoglobin das Discoplasma vorhanden ist, 



