The formation of the first blood cells which develop in the area vasculosa of the chick embryo have been examined to seven days of incubation. Angioblasts have been seen to arise from the mesoderm and to give origin to the first endothelium and intravascular blood islands. This occurs as the intercellular spaces enlarge and coalesce. The angioblasts and blood islands are not syncytial but are held in close relation by attachment areas and cytoplasmic processes. Two cell types arise concurrently from the blood islands, a stem cell and cells which have begun their differentiation into the primitive red cell series. The morphological events associated with this differentiation are similar to those in other hematopoietic organs. The definitive series of red blood cells arises from the stem cells at 4 to 5 days of incubation. The morphological differentiation of the definitive red cell is generally similar to that of the primitive series although the late and mature stages show structural differences. Nuclear structure, relation of interchromatin material to nuclear pores and the presence of marginal bands in the definitive series allow a differentiation between the two series.

Granulocytes arise from extravascular stem cells at about five days of incubation but at seven days are not fully mature or abundant.

Limited observations on the avian thrombocyte are reported.