[Origin of the adenohypophysis and of the olfactory system in toads: demonstration by chimera method]

Localisation of the primordium of the olfactory system and its topographical relationships with the primordium of the hypothalamic-hypophyseal system were determined on the embryos of the toad Bufo japonicus by generating chimeric embryos. Different parts of the neural ridge and the neural plate were taken up from normal embryos and grafted on the same region of albino embryos of the same developmental stage. The melanin granules contained in the grafts permitted to trace the developmental fate. The anteriormost region of the neural ridge (ANR) was found to be the origin of all the parts of the adenohypophysis as well as some of the neurons in the preoptic region. The infundibulum differentiated from the central portion of the neural plate, just posterior to the adenohypophyseal primordium. The primordium of the olfactory system, including the olfactory sacs and the olfactory bulbs, was found to be situated on the neural ridge on both sides of the ANR. The present results showed that the central and peripheral parts of each of the two systems constitute a closely apposed cell population at the neurula stage. It seems that the periphero-central connection in these systems reflect a clonal origin of the presumptive cells. If the olfactory primordium was removed at the neurula stage, the olfactory sac regenerated from the adenohypophyseal primordium. In conclusion, these results demonstrate that the olfactory primordium is related not only topographically but also ontogenetically to the adenohypophyseal primordium.