34
TUNEO
YAMADA
That the neural induction obtained in the last mentioned experiments
of Brächet was probably of archencephalic type, is implied by the
following results of Kuusi (1953). The homogenate oí Triturus embryos
in phosphate buffer was centrifuged at 3000 rev/min for 10 minutes.
The supernatant and residue were tested after ethanol treatments by
the insertion technique. Archencephalic or non-regional inductions were
obtained with both fractions, the frequency of the archencephalic
structures being higher in the supernatant fraction which contained the
major part of RNA.
VI. RNA and Specific Induction
In 1953 Niu and Twitty reported an important observation concerning
the hanging drop culture of ectoderm cells of several urodeles. The cells
from the presumptive ectoderm area differentiate into neural cells and
chromatophores, when cultured in medium in which organizer cells have
been developing for one week or more. The induction does not require
the simultaneous presence of organizer cells in the medium. No differentiation other than cilia formation occurs when the cells are cultured in
'unconditioned' medium, or are introduced into the fresh medium with
the organizer cells. When the ectoderm cells are cultured in medium in
which somites have been developing for 14^-18 days, they form myoblasts.
According to a later paper of Niu (1956) the presence of nucleoprotein
was indicated by the absorption spetrum of the concentrated 'conditioned' medium. Colour reactions for RNA, DNA, and protein were
positive. Evidence was presented for the non-dialyzable and ethanolprecipitable nature of the active substance. Niu studied the effect of
ribonuclease, trypsin, and chymotrypsin on the inducing ability of the
'conditioned' medium. In experiments with Triturus torosus and
Ambystoma mexicanum ectoderm the ribonuclease prevented the
induction. However, a definite conclusion was difficult to draw, because
in the experiments with the Triturus rivularis ectoderm proteases but
not nucleases prevented induction. Furthermore, as Niu admits, in these
experiments it was possible that the enzymes attacked the cells directly
and influenced their differentiation. Altogether, these experiments made
clear that induction of neural and mesodermal differentiation is made
possible by a factor or factors released from developing cells of the
organizer area, and the factor or factors are macromolecules, and
possibly nucleoproteins. However, the question has not been answered
whether the factors operative in the experiment are identical with the
normal agents of the organizer, or are a product of differentiated
mesoderm cells derived from the organizer cells, having no direct
relation with the organizer action.
TUNEO
YAMADA
That the neural induction obtained in the last mentioned experiments
of Brächet was probably of archencephalic type, is implied by the
following results of Kuusi (1953). The homogenate oí Triturus embryos
in phosphate buffer was centrifuged at 3000 rev/min for 10 minutes.
The supernatant and residue were tested after ethanol treatments by
the insertion technique. Archencephalic or non-regional inductions were
obtained with both fractions, the frequency of the archencephalic
structures being higher in the supernatant fraction which contained the
major part of RNA.
VI. RNA and Specific Induction
In 1953 Niu and Twitty reported an important observation concerning
the hanging drop culture of ectoderm cells of several urodeles. The cells
from the presumptive ectoderm area differentiate into neural cells and
chromatophores, when cultured in medium in which organizer cells have
been developing for one week or more. The induction does not require
the simultaneous presence of organizer cells in the medium. No differentiation other than cilia formation occurs when the cells are cultured in
'unconditioned' medium, or are introduced into the fresh medium with
the organizer cells. When the ectoderm cells are cultured in medium in
which somites have been developing for 14^-18 days, they form myoblasts.
According to a later paper of Niu (1956) the presence of nucleoprotein
was indicated by the absorption spetrum of the concentrated 'conditioned' medium. Colour reactions for RNA, DNA, and protein were
positive. Evidence was presented for the non-dialyzable and ethanolprecipitable nature of the active substance. Niu studied the effect of
ribonuclease, trypsin, and chymotrypsin on the inducing ability of the
'conditioned' medium. In experiments with Triturus torosus and
Ambystoma mexicanum ectoderm the ribonuclease prevented the
induction. However, a definite conclusion was difficult to draw, because
in the experiments with the Triturus rivularis ectoderm proteases but
not nucleases prevented induction. Furthermore, as Niu admits, in these
experiments it was possible that the enzymes attacked the cells directly
and influenced their differentiation. Altogether, these experiments made
clear that induction of neural and mesodermal differentiation is made
possible by a factor or factors released from developing cells of the
organizer area, and the factor or factors are macromolecules, and
possibly nucleoproteins. However, the question has not been answered
whether the factors operative in the experiment are identical with the
normal agents of the organizer, or are a product of differentiated
mesoderm cells derived from the organizer cells, having no direct
relation with the organizer action.