ABSTRACT
Presumptive neural plate ectoderm of an early gastrula, when separated by a porous membrane (average pore diameters, 4 mμ) from the axial mesoderm, failed to differentiate into neural tube either in Axolotl or in Xenopus.
In ectodermal sandwich experiments induction always occurred on the side containing the organizer in Triturus alpestris and was prevented by the interposition of a porous membrane, when the latter had an average pore diameter of 4 mμ or 1· 45 μ.
No cytoplasmic connexion across the membrane could be observed.
The lack of induction through the membranes might be due either to a total failure of diffusion of an active substance, or to a failure of a diffusing substance to attain a high enough concentration on the other side of the membrane, or the induction may essentially involve cell-to-cell contact. No decision between these alternatives is reached.
