Proliferation and differentiation processes of chick embryo primary erythroid cells (PEC) were studied. A novel differentiation pathway was discovered by which cells of proerythroblastic and erythroblastic stages are blocked in G1 or G2 phases, to develop then directly into reticulocytes, i.e. terminally differentiated non-dividing cells with high hemoglobin contents differing in shape from erythrocytes. These cells appear in blood two days earlier than erythrocytes, then they co-exist with the latter and are eliminated in parallel with them. This pathway leads to a rapid enrichment of PEC with hemoglobin. A fraction of PEC forms accessory nuclei, which, as it is shown here, contain an extra quantity of DNA. Compared to the diploid ones, such cells reveal increased hemoglobin contents which enabled us to assume that they may have amplified the globin genes. The above-mentioned pecularities of cytodifferentiation may be presumably an adaptation to oxygen supply of growing embryos which are known to stay in hypoxia. A comparison of these results with results of our earlier study on experimental anemia makes it possible to suppose that pecularities of these two types of cytodifferentiation may be based on similar or, perhaps, analogous mechanisms of regulation.