Abstract
Phallusia mamillata embryos at the 8-, 16-, 32- and 61-cell stages have been deprived of determined vegetative anterior blastomeres (deriving from, the A4.1 cells), with the purpose to sec which influence is exerted by such blastomeres in the induction of the nervous system. The results have been the following. 1 — The nervous system is not yet determined at the 64-cell stage. 2 — A neurali induction can be exerted even by a few blastomeres of the 64-ccll stage, containing cither entodermic or chordal material. 3 — Such inducing blastomeres arc those which give rise to the archenteron roof of the gastrula.

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