The erythropoietic area of very early chick embryos was cultured as a tissue for up to nine days to study the changes in red cell type and hemoglobin type, the cell cycle time, the cell population kinetics, and the DNA synthetase activity of these cells. It was found that the area vasculosa without the participation of the embryo proper contained the information and the timing mechanism required to produce not only the early primitive erythroid cell population, but also in due course, the later definitive cell type, each with its appropriate hemoglobin types. Also the precursors of the definitive cell type are active in DNA synthesis and therefore are probably in cycle very early in the culture period.

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