Separation of an anterior inducing activity from development of dorsal axial mesoderm in large-headed frog embryos.

The body of a vertebrate arises through a series of inductive interactions in the embryo. Macrocephaly is a distortion of the body in which a disproportionate amount of tissue is devoted to the head. This syndrome occurs in certain hybrids between frog species and appears to be due to an alteration of inductive relationships. Chimeric blastulae between normal and hybrid embryos developed macrocephaly when the marginal zone was derived from the hybrid. In these cases, a large cement gland, characteristic of the hybrid head, was induced to form from normal ectoderm. When hybrid zygotes were irradiated with ultraviolet (uv) light, all dorsoanterior structures, including notochord, somites, and central nervous system, were eliminated, but the most anterior-induced structure, the cement gland, remained. Embryos without dorsoanterior structures but with cement glands were also produced by injecting germinal vesicle extracts into the blastocoel of uv-irradiated nonhybrid embryos. These results demonstrate that an anterior inducing activity can be uncoupled from development of the neural tube and dorsal axial mesoderm.