M O R P H O G E N E S I S OF F R E S H - W A T E R P L A N A R I A
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Probably, therefore, regeneration of the brain takes place in the
following manner. After decapitation of the planarian, the regeneration
bud of the head builds up and differentiates a brain at the expense of the
neoblasts in contact with the nerve trunks. The brain, once regenerated,
emits inhibitory substances which prevent the differentiation of another
cerebral mass.
4. Regeneration of the Eyes
The regeneration of the eyes is often utilized as a test of the differentiation of the head of fresh-water planarians. The differentiation of the
structures of the head is completed when the eyes appear.
The eye of the planarians has a very simple structure. A pigmented
ocular cup and visual cells can be distinguished. The visual cells form
at one of the poles of the visual cones inserted in the ocular cup while the
other pole sends out a nerve ending which is connected to the brain by
the optic nerve (Fig. 12).
Regeneration of the eye takes place by differentiation of the neoblasts.
First the ocular cup forms, then the visual cells appear, and, lastly,
communication with the brain is effected.
The determinism of the regeneration of the eyes has been studied in
FIG. 12. Transverse section of the eye of Dugesia lugubris. C: brain; CO: ocular cup;
CV: visual cone; N : nucleus of a visual cell.