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differentiation of the eyes. It would, of course, be of great interest to
discover the chemical nature of these substances.
In any event, in fresh-water planarians the nervous system does not
play merely a trophic or stimulatory role as had been thought formerly.
The brain is indispensable for the normal unfolding of regeneration since
it intervenes directly in the differentiation of certain organs and in the
realization of the form of the organism.
If we now analyse the normal course of regeneration, we note that the
fate of the blastema depends on its location in the organism. From the
second day of regeneration this determination is fixed irrevocably.
The organs differentiate from the blastema which is built up by
FIG. 14. Schematic representation of the role of inducers and inhibitors during
regeneration of fresh-water planarians. The inductive and inhibitory activities are
indicated by arrows, a: isolation of the caudal region; b : regeneration of the brain (CE)
in the blastema, emission of inhibitors of the differentiation of another brain; c: induction of the regeneration of the eye (O) by the 'organisines' emitted by the brain, constitution of the cephalic region (T); d: induction of the prepharyngeal region (PR) by the
head; e and f: induction of the pharyngeal region (PH) by the prepharyngeal region;
g: induction of the pharynx (Ph) by the pharyngeal region; h: emission of hormones by
the testes (TE). (Wolff, 1953, Fig. 3 modified.)