Blastopore formation inAmbystoma mexicanum.

UsingAmbystoma mexicanum blastulae (Harrison 7 2/3 to 10-), a study was made of the morphogenetic behaviour of the endoderm, both in isolation and upon recombination with marginal zone material.It was shown that the endoderm autonomously forms "flask cells" and thus a blastoporal groove, when isolated at or after the mid-blastula stage (8+). The frequency of blastopore formation decreases as the endoderm is isolated at successively younger stages. At the youngest operable stage (7 2/3) a blastoporal groove is formed in 30% of the isolates.Recombination with marginal zone material promoted blastopore formation in endoderm isolated at stage 7 2/3; the frequency of blastopore formation was maximal (90%) in recombinates with dorsal marginal zone from stages 7 2/3 and 8 1/2, while in recombinates with older dorsal (stages 9 and 10-) or with ventral marginal zone (stages 7 2/3 to 10-) it was lower (60%). Blastopore formation could only be evoked on the dorsal side of the endoderm.Ventral halves of stage 10- endoderm are unable to form flask cells, whereas the complete endoderm forms flask cells ventrally as well as dorsally, demonstrating the leading role of the dorsal endoderm in flask cell formation.Only the left half of the dorsal or ventral marginal zone was used in the recombinates, the corresponding right half being culturedin vitro. Differentiation of mesoderm in these control explants was used as a criterion for the presence of induced mesoderm in the marginal zone material at the time of recombination. Differentiation was poor, however, which affects the reliability of this criterion. No relation could be found between the presence of induced mesoderm in the isolated marginal zone material and the incidence of gastrulation phenomena in the endoderm/marginal zone recombinates.Thus the results of these experiments provide no evidence for a gastrulation-inducing role of the induced mesoderm, as proposed by Nieuwkoop (1969a). Alternatively, the results suggest that the endoderm may acquire its capacity for blastopore formation as a direct consequence of the earlier dorso-ventral polarization of the embryo, without the intervention of the induced mesoderm. Synchronization of endodermal and mesodermal gastrulation movements must be established during mesoderm induction.