320
TH. LENDER
Sexualization was not obtained if the anterior region of the sexual form
was used before sexual maturity.
In the inverse experiment, Okugawa grafted the anterior region of an
asexual form to the posterior region of a sexual form. Ovaries and testes
differentiated in the anterior region.
The asexual form does not regenerate genital organs in the presence
of a graft of brain or of tissue devoid of ovary and testis derived from the
sexual form.
Okugawa proposed that the induction of genital organs is related to
the presence of testes which may emit sex hormones of as yet unknown
nature. But in these experiments one cannot eliminate completely the
hypothesis of the build-up of the genital apparatus from neoblasts
migrating from the sexual graft in the asexual region provided that the
graft contains enough neoblasts. These experiments may be compared
with the results obtained by Hauenschild (1954, 1956, 1957) and
Weiler-Stolt (1960) on the medusa Eleutheria dichotoma. Hauenschild
raised asexual clones which in only 1-9% of cases were able to recover
their sexuality. The transplantation of tissues of the sexual clone to the
asexual clone caused the induction of sexuality provided that the graft
contained interstitial cells which could undergo sexual differentiation.
These cells furnish the material of the incubation cavity and the sexual
cells of the medusa in which the sexual potentialities have been lost or
repressed. The interstitial cells play the same role during regeneration of
the coelenterates as the neoblasts do in the planarians.
3. Regeneration and Inhibition of the Brain
If a planarian is decapitated, the organ which differentiates first is the
brain. At the present time, nearly all authors agree that the nerve cells
are derived from the differentiation of the neoblasts in contact with the
nerve trunks which branch out at the level of the regeneration blastema.
However, Bondi (1959) assumes that the brain forms by proliferation of
nerve trunks without the intervention of neoblasts. In the blastema, the
cerebral ganglia differentiate and develop at the same time as growth
proceeds.
But this regeneration does not occur in the presence of another brain.
An inhibition is observed which prevents the differentiation of a head.
The inhibitory activity of the planarian brain has been demonstrated by
Morgan (1902), Rand and Ellis (1926), Chevtchenko (1937), and Miller
(1938). Chevtchenko (1937) found that a head graft transplanted in the
cephalic region of the host is inhibited in its growth. The inhibitory
activity is manifested only in the anterior region of the planarian. It
diminishes in intensity and then disappears toward the caudal region.
The inverse experiment was carried out by Lender (1960). He grafted a
TH. LENDER
Sexualization was not obtained if the anterior region of the sexual form
was used before sexual maturity.
In the inverse experiment, Okugawa grafted the anterior region of an
asexual form to the posterior region of a sexual form. Ovaries and testes
differentiated in the anterior region.
The asexual form does not regenerate genital organs in the presence
of a graft of brain or of tissue devoid of ovary and testis derived from the
sexual form.
Okugawa proposed that the induction of genital organs is related to
the presence of testes which may emit sex hormones of as yet unknown
nature. But in these experiments one cannot eliminate completely the
hypothesis of the build-up of the genital apparatus from neoblasts
migrating from the sexual graft in the asexual region provided that the
graft contains enough neoblasts. These experiments may be compared
with the results obtained by Hauenschild (1954, 1956, 1957) and
Weiler-Stolt (1960) on the medusa Eleutheria dichotoma. Hauenschild
raised asexual clones which in only 1-9% of cases were able to recover
their sexuality. The transplantation of tissues of the sexual clone to the
asexual clone caused the induction of sexuality provided that the graft
contained interstitial cells which could undergo sexual differentiation.
These cells furnish the material of the incubation cavity and the sexual
cells of the medusa in which the sexual potentialities have been lost or
repressed. The interstitial cells play the same role during regeneration of
the coelenterates as the neoblasts do in the planarians.
3. Regeneration and Inhibition of the Brain
If a planarian is decapitated, the organ which differentiates first is the
brain. At the present time, nearly all authors agree that the nerve cells
are derived from the differentiation of the neoblasts in contact with the
nerve trunks which branch out at the level of the regeneration blastema.
However, Bondi (1959) assumes that the brain forms by proliferation of
nerve trunks without the intervention of neoblasts. In the blastema, the
cerebral ganglia differentiate and develop at the same time as growth
proceeds.
But this regeneration does not occur in the presence of another brain.
An inhibition is observed which prevents the differentiation of a head.
The inhibitory activity of the planarian brain has been demonstrated by
Morgan (1902), Rand and Ellis (1926), Chevtchenko (1937), and Miller
(1938). Chevtchenko (1937) found that a head graft transplanted in the
cephalic region of the host is inhibited in its growth. The inhibitory
activity is manifested only in the anterior region of the planarian. It
diminishes in intensity and then disappears toward the caudal region.
The inverse experiment was carried out by Lender (1960). He grafted a