Summary

1. The just formed side-plate mesoderm from the gastrula in the medium-sized yolk-plug stage can induce the formation of an extra neural plate when implanted into the blastocoel of a young gastrula.

2. The dorsal lip of a young gastrula, substituted for the side-plate mesoderm mentioned in (1), evocates a neural plate.

3. The failure of evocation by the side-plate mesoderm in normal development can only be explained by supposing both that the evocating capacity of the side-plate mesoderm is lower than that of the dorsal lip and that the lateral ectoderm does not react to an evocating stimulus as easily as does the young gastrula ectoderm.

4. The weakness of the evocating capacity of the side-plate mesoderm might be due either to its intrinsic nature or to an active suppression by the rest of the mesoderm. The evidence suggests that both factors may be involved, but it can be shown that the latter factor by itself is not sufficient to explain the facts.

5. Isolated gastrula ectoderm retains its competence to be evocated to neural tissue at least until the age of an open neural plate embryo. Thus the normal loss of competence by the lateral ectoderm is not due to an inherent process. Nor is it caused solely by a continued connection with the neural plate. It cannot be due merely to the presence of side-plate mesoderm; but it may be due to an influence proceeding from side-plate mesoderm which is part of a mesoderm field, or it may be dependent on a preparation of the ectoderm by reason of its connection with the neural plate, or, more probably, by an inherent process, so that by the stage at which the side-plate mesoderm forms, the ectoderm has become such that it reacts with side-plate mesoderm with rapid loss of competence.