“…The presence of endoderm though not absolutely required for erythro cyte differentiation does improve the eryth ropoietic performance of the tissue [4], Erythropoiesis in chick embryos has been studied extensively in vivo [1,3,[5][6][7][8]. In vitro culture of unincubated chick blasto derm in which the primitive streak, and con sequently the embryonic body are inhibited mechanically, can form primitive and defini tive erythrocytes and embryonic and adult Hbs as in development in ovo [9][10][11].…”