Differentiation potential of posterior streak mesoderm. (A-D) HH4 streak pieces are grafted to the lateral area opaca of host HH4 embryo and cultured to HH10 (A). Differentiation is assayed by ρ expression for anterior (B), middle (C), and posterior (D) pieces. Red dotted lines indicate the limit of host ρ expression. (E-H) HH4 streak pieces are cultured on vitelline membrane (E). Differentiation is assayed by ρ when control reaches HH10 (F) for posterior (G) and third (H) quarters.