DORSAL SPECIFICATION IN BLASTODERM AT THE BLASTULA STAGE IN THE GOLDFISH, CARASSIUS-AURATUS

The teleost dorsoventral axis cannot be morphologically distinguished before gastrulation. Previous studies by the current authors have show n that localized dorsalizing activity in the yolk cell (YC) induces th e dorsal tissues in the overlying blastoderm. In order to examine whet her or not dorsal blastomeres are committed to their dorsal fate befor e the gastrula stage, a variety of transplant operations were performe d in goldfish blastoderms at the mid- to late-blastula stages. When th e blastoderm was cut from the YC, rotated horizontally at 180 degrees, and recombined with the YC, the blastoderm frequently developed two a xes, indicating that dorsal blastomeres of the blastula had already ac quired the ability to differentiate into the organizer in the absence of dorsalizing signals from the YC. This result was further confirmed by experiments using ventralized embryos in which no dorsal structures formed. the axis formation was frequently observed in the normal blas toderm combined with the ventralized YC at the blastula stage. However , the axes formed in the absence of dorsal information from the YC exh ibited a lower dorso-anterior index. Furthermore the dorsal specificat ion was not stably maintained when the dorsal cells were located far f rom the YC. These results suggest that the inductive and permissive in fluence of the YC may be required for the blastoderm to undergo full d orsal differentiation.