An analysis of the postgastrula differentiation of the hypomere. I. The influence of tissue mass and of endoderm in Ambystoma punctatum.

T H E initial report in this series (Finnegan, 1961) emphasized the role of the endoderm in the postgastrula differentiation of the hypomeric mesoderm in Ambystoma punctatum. The effect of the endoderm appeared to be modified when the mass of mesoderm involved was increased and, under the in vitro experimental conditions employed, the endoderm did not influence the splanchnic layer of the hypomere into new types of histogenesis (induction). Thus it was concluded that the endoderm aided the histogenesis of the splanchnic mesoderm in its vicinity in a synergistic manner rather than as an inductive tissue. Further evidence of the mechanism of this assistance by the endoderm of the postgastrula development of the mesoderm has been obtained from similar in vitro studies with tissues from Taricha torosa neurulae. This report is concerned with results which substantiate the previously derived conclusions and, further, indicate at least one manner in which the endoderm effects its synergistic role.