It has been reported that the micromeres of echinoid embryos have the potential to induce an archenteron in animal cap mesomeres recombined at the 16- or 32-cell stage. In the present study, experiments were performed to determine the exact period when the micromeres transmit their inductive signal to respecify the cell fate of mesomeres as endo-mesoderm. An animal cap was recombined with a quartet of micromeres, or micromere-descendants cultured in isolation, to form a recombinant embryo. The micromere- descendants were completely removed at various developmental stages, resulting in an embryo composed only of mesomere-descendants that had been under the inductive influence of micromeres for a limited period. The resulting embryos were cultured and examined for their potential to differentiate endoderm. The results indicated that the signal effective for inducing an archenteron in mesomere-descendants emanated from the micromere- descendants at the early blastula stage around hatching onward. Before this stage, the micromeres and micromere-descendants showed this potential slightly or not at all. The inductive signal emanated from the micromere- descendants almost on time even when the cells were cultured in isolation. The micromere-descendants completed transmission of the signal for inducing the archenteron in the animal cap within 2h of recombination. The animal cap at between the 28-cell stage and 2 h after the 32-cell stage could react with the inductive signal from the micromere-descendants. Embryos composed of only animal cap mesomeres that had received the inductive signal from micromere- descendants for a limited period had the potential to develop into 8-armed plutei. Each pluteus formed an adult rudiment essentially on the left side of the larval body, and metamorphosed into a juvenile with pentaradiate symmetry.
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