MORPHOGENESIS OF FRESH-WATER PLANARIA
311
twelfth and the fifteenth day after the operation it was no longer possible
to detect the neoblasts by methyl green-pyronine staining. Soon afterwards the planarian decomposed. Having repeated the experiment of
Dubois with localized irradiation and decapitation, Lender and Gabriel
(1961) found that the neoblasts had ceased to exist in the irradiated
zone. The regeneration blastema did not form immediately. A little later,
undamaged regeneration cells were seen to reappear in the irradiated
zone behind the wound (Fig. 5). The entire irradiated zone then became
populated by these cells and a regeneration blastema was formed by the
accumulation of DNA- and RNA-containing neoblasts. The regeneration bud appeared only after colonization of the irradiated zone by
healthy neoblasts migrating from non-irradiated regions in the body of
the planarian.
The neoblasts, therefore, react in two different ways to the stimulus
produced by the wound. The cells nearest the wound begin to migrate
at once while the more distant ones begin to multiply actively and
migrate later. The nature of the stimulus is not yet known. But the
question arises as to the manner in which it acts to cause the migration
and division of the neoblasts. Dubois (1949) put forward two hypotheses,
either it acted through the nervous system or by the emission of
chemical sustances in the body fluids of the planarian.
These two hypotheses are probably valid, as shown by the study of
neurosecretion in the planarians (Lender and Klein, 1961). Outside of
periods of regeneration, histochemical techniques have shown that
neurosecretory cells exist in the brain of Poly cells nigra and, to a small
extent, along the nerve trunks. The fuchsin-paraldehyde method of
Gabe shows that these cells are quite abundant in the middle and
posterior region of the brain (cerebral commissure).
When a planarian regenerates a posterior region, there is a very
pronounced increase in the number of neurosecretory cells in the brain
during the three days which follow the cutting. It may therefore be
postulated that neurosecretion is involved in regeneration.
C. Planarian Neoblasts with Limited Regeneration Capacity
It is known that the power of regeneration may be more or less
limited in certain species of fresh-water planarians. The frequency curve
of regeneration of the head as a function of the location of the cut varies
according to the species, as shown by Sivickis (1931) and Br0ndsted
(1939). Dendrocoelum lacteum has a limited regeneration capacity. The
rate of regeneration of the head and the quality of the regenerated
portion depends on the location of the cut; decapitation behind the eyes
is followed by normal regeneration. If the cut is made half-way between
the eyes and the fundus of the pharynx, the regeneration of the head is
311
twelfth and the fifteenth day after the operation it was no longer possible
to detect the neoblasts by methyl green-pyronine staining. Soon afterwards the planarian decomposed. Having repeated the experiment of
Dubois with localized irradiation and decapitation, Lender and Gabriel
(1961) found that the neoblasts had ceased to exist in the irradiated
zone. The regeneration blastema did not form immediately. A little later,
undamaged regeneration cells were seen to reappear in the irradiated
zone behind the wound (Fig. 5). The entire irradiated zone then became
populated by these cells and a regeneration blastema was formed by the
accumulation of DNA- and RNA-containing neoblasts. The regeneration bud appeared only after colonization of the irradiated zone by
healthy neoblasts migrating from non-irradiated regions in the body of
the planarian.
The neoblasts, therefore, react in two different ways to the stimulus
produced by the wound. The cells nearest the wound begin to migrate
at once while the more distant ones begin to multiply actively and
migrate later. The nature of the stimulus is not yet known. But the
question arises as to the manner in which it acts to cause the migration
and division of the neoblasts. Dubois (1949) put forward two hypotheses,
either it acted through the nervous system or by the emission of
chemical sustances in the body fluids of the planarian.
These two hypotheses are probably valid, as shown by the study of
neurosecretion in the planarians (Lender and Klein, 1961). Outside of
periods of regeneration, histochemical techniques have shown that
neurosecretory cells exist in the brain of Poly cells nigra and, to a small
extent, along the nerve trunks. The fuchsin-paraldehyde method of
Gabe shows that these cells are quite abundant in the middle and
posterior region of the brain (cerebral commissure).
When a planarian regenerates a posterior region, there is a very
pronounced increase in the number of neurosecretory cells in the brain
during the three days which follow the cutting. It may therefore be
postulated that neurosecretion is involved in regeneration.
C. Planarian Neoblasts with Limited Regeneration Capacity
It is known that the power of regeneration may be more or less
limited in certain species of fresh-water planarians. The frequency curve
of regeneration of the head as a function of the location of the cut varies
according to the species, as shown by Sivickis (1931) and Br0ndsted
(1939). Dendrocoelum lacteum has a limited regeneration capacity. The
rate of regeneration of the head and the quality of the regenerated
portion depends on the location of the cut; decapitation behind the eyes
is followed by normal regeneration. If the cut is made half-way between
the eyes and the fundus of the pharynx, the regeneration of the head is