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TH. LENDER
Polycelis nigra (Lender, 1952a, b). This planarian possesses numerous
marginal eyes some distance from the brain. The regeneration of the
anterior eyes is more rapid than that of the more posterior lateral eyes
(Br0ndsted 1942). Wolff and Lender (I950a,b) showed that the
regeneration of the eyes was induced by the brain. The brain oi Polycelis
nigra was excised and its regeneration was prevented by destroying the
blastema every other day. The excised lateral eyes did not differentiate.
In these experiments, the excision of the brain was accompanied by
the suppression of the optic nerves which issue from it and by the
elimination of the neoblasts located nearby.
The absence of nerve connections did not disturb the regeneration of
the eyes since the tie between the brain and the eye is not established
until the end of the period of differentiation.
Eyes may also regenerate in the absence of neoblasts which are in
contact with the brain. X-irradiation of the anterior region of the
planarian according to the technique of Wolff and Dubois (1947a, b, c)
destroys these neoblasts. In their absence, the eyes regenerate from
healthy neoblasts, migrating from the non-irradiated region, which have
not been in contact with the brain.
It must therefore be assumed that the brain exerts an inductive
influence at a distance which is the cause of the differentiation of the
ocelli. Bondi (1958a) also postulates an inductive activity of the brain
in the regeneration of the eyes of Dugesia lugubris.
It can be shown that the brain is surrounded by an inductive field
(Lender, 1950, 1951a). If a graft of the eye-bearing rim is implanted
behind the brain, the excised eyes regenerate and are even more
numerous than before. If the graft is implanted in the pharyngeal
region, regeneration of the eyes is still possible, but a graft placed in the
caudal region does not lead to regeneration. The inductive field of the
brain, therefore, extends to the pharyngeal region.
In the postpharyngeal region this field does not exist but it can be
created by implanting a brain graft next to the graft of the eye-bearing
rim with excised eyes. Regeneration of the eyes is produced (Fig. 13).
The inductive activity of the brain during the regeneration of the eyes
is independent of the histological integrity of nervous tissue (Lender,
1951b). A brain subjected to X-irradiation becomes necrotic 30 to 40
days after exposure. At this point, only a few nerve cells of healthy
appearance are to be seen. But this irradiated brain still exhibits its
capacity to induce the differentiation of the eyes. However, this
induction does not manifest any zoological specificity. If, after the
excision of the eyes, an eye-bearing rim of Polycelis nigra is implanted
near the brain of Dugesia lugubris, it also regenerates eyes. In this
instance, there are no nerve connections and no migration of neoblasts
TH. LENDER
Polycelis nigra (Lender, 1952a, b). This planarian possesses numerous
marginal eyes some distance from the brain. The regeneration of the
anterior eyes is more rapid than that of the more posterior lateral eyes
(Br0ndsted 1942). Wolff and Lender (I950a,b) showed that the
regeneration of the eyes was induced by the brain. The brain oi Polycelis
nigra was excised and its regeneration was prevented by destroying the
blastema every other day. The excised lateral eyes did not differentiate.
In these experiments, the excision of the brain was accompanied by
the suppression of the optic nerves which issue from it and by the
elimination of the neoblasts located nearby.
The absence of nerve connections did not disturb the regeneration of
the eyes since the tie between the brain and the eye is not established
until the end of the period of differentiation.
Eyes may also regenerate in the absence of neoblasts which are in
contact with the brain. X-irradiation of the anterior region of the
planarian according to the technique of Wolff and Dubois (1947a, b, c)
destroys these neoblasts. In their absence, the eyes regenerate from
healthy neoblasts, migrating from the non-irradiated region, which have
not been in contact with the brain.
It must therefore be assumed that the brain exerts an inductive
influence at a distance which is the cause of the differentiation of the
ocelli. Bondi (1958a) also postulates an inductive activity of the brain
in the regeneration of the eyes of Dugesia lugubris.
It can be shown that the brain is surrounded by an inductive field
(Lender, 1950, 1951a). If a graft of the eye-bearing rim is implanted
behind the brain, the excised eyes regenerate and are even more
numerous than before. If the graft is implanted in the pharyngeal
region, regeneration of the eyes is still possible, but a graft placed in the
caudal region does not lead to regeneration. The inductive field of the
brain, therefore, extends to the pharyngeal region.
In the postpharyngeal region this field does not exist but it can be
created by implanting a brain graft next to the graft of the eye-bearing
rim with excised eyes. Regeneration of the eyes is produced (Fig. 13).
The inductive activity of the brain during the regeneration of the eyes
is independent of the histological integrity of nervous tissue (Lender,
1951b). A brain subjected to X-irradiation becomes necrotic 30 to 40
days after exposure. At this point, only a few nerve cells of healthy
appearance are to be seen. But this irradiated brain still exhibits its
capacity to induce the differentiation of the eyes. However, this
induction does not manifest any zoological specificity. If, after the
excision of the eyes, an eye-bearing rim of Polycelis nigra is implanted
near the brain of Dugesia lugubris, it also regenerates eyes. In this
instance, there are no nerve connections and no migration of neoblasts