Differentiation of the hematopoietic tissue of the yolk sac membrane of chick embryos during the first 6 days of development was observed by light and electron microscopy. Specific cytoplasmic and nuclear alterations were observed in primitive line erythroid maturation stages from presumptive precursor cells to mature erythrocytes. Particularly significant was the increasing aggregation of condensed chromatin and its organization within the nucleus during erythropoiesis. Electron microscope observations at 2 to 5 days of incubation suggested that the intravascular erythroblast population is heterogeneous, 2 or more maturation stages coexisting within the embryonic circulation on any particular day.

Definitive line erythroid cells were not present in the embryonic circulation until approximately 5 days of incubation, but presumed definitive line precursor cells were present as sequestered populations possibly as early as 2 days of incubation.

Chromatin condensation was observed to be an early event during erythroid cell maturation and is possibly a preprogrammed part of differentiation of this cell type. During maturation of primitive line erythroid cells, there was no marked change in the volume of condensed chromatin (on a per genome basis). Decreasing nuclear volume and increasing aggregation of condensed chromatin masses during differentiation combine to produce the illusion of increasing chromatin condensation observed in nuclear profiles in cytological preparations. There was no change in the density of fibril packing in condensed chromatin at different maturation stages.

Cultures of two cell populations prepared by density-gradient centrifugation of dissociated early chick embryos, yielded discrete islands of erythroid cells within a large mass of endodermal cells. One population contained erythroid precursor cells with a greater potential for multidirectional differentiation than the second population, though both types of precursor cell were capable of differentiating into maturing erythroblasts resembling  comparable maturation stages noted in ovo.