The roles of cell cycles and of cell-cell interactions in the emergence of myogenic and endothelial cell lineages were studied in avian embryos using the quail-chicken marker system. Quail embryos were treated with drugs preventing either DNA replication or the movement of cells. Portions of drug-treated or untreated quail blastoderms were grafted into chicken wing buds. After an incubation for an additional 4 to 10 days, the embryos were analyzed for the presence of quail muscle or quail endothelial cells by the Feulgen reaction and by immunostaining. Both cell lineages differ in the time of their commitment as well as in the conditions necessary for their emergence. Muscle cells did not differentiate from unincubated blastoderms nor did they develop from drug-treated blastoderms. These results corroborate that the commitment of myogenic cells occurs during gastrulation and indicate that this commitment requires both DNA replication and cellular movements allowing cell-cell and/or cell-matrix interactions. Endothelial cells, on the contrary, developed both from drug-treated and from unincubated blastoderms, indicating that their commitment occurs before and independent of gastrulation and does not require DNA replication during gastrulation.