Morphogenetic interactions before gastrulation in the amphibian Xenopus laevis. The cortical

T H E site of the future dorsal lip of the gastrula is already determined to a con-siderable degree at the grey-crescent region by the 8-cell stage, probably by means of interactions within the cortex (Curtis, 19626). But this determination is still easily disturbed because excision of the grey-crescent cortex from the 8-cell stage does not prevent the appearance of a dorsal lip. The embryo forms a new dorsal lip by a process of regulation. Similarly, Votquenne (1933) destroyed the blasto-mere containing the grey crescent at the 8-cell stage in Ranafusca and neverthe-less obtained nearly normal embryos; which result indicated that regulation had happened. At a later stage, late blastula, excision of the cells of the presumptive dorsal Up region does not prevent a dorsal lip forming and subsequent neural induction occurring (Goerttler, 1926) although the embryos are of the abnormal 'ring ' type. But though regulation in Goerttler's experiments can hardly have occurred before gastrulation, due to the late stage used, it remains an open question whether the regulation of the embryo after Votquenne's excision of a micromere or my excision of cortex from the 8-cell stage embryo, happens soo