During the first few days of incubation the events concerning erythropoiesis occur rapidly but there is a recognized significant variation in the rates that different chick embryos develop. The erythrocytic progenitors of a formed blood island are less closely packed than the preceding, less differentiated hemangioblasts, but they exhibit cell junctions. A specific feature that distinguishes the proerythroblasts from blood island cells and seems to further indicate their active commitment to the erythrocytic lineage is the occurrence of many cytoplasmic electron-dense bodies that may contain apoferritin and ferritin and which are apparently involved in iron metabolism. The formation of the earliest blood cells of the chick embryo has been subjected to detailed ultrastructural analyses. A landmark in the ontogeny of the erythrocyte that is documentable at this juncture is that most, if not all, cells released from the blood islands spectrophotometrically reveal that they contain hemoglobin.