40
TUNEO
YAMADA
also be taken up by the ectoderm cells by pinocytosis. No study has yet
been made to test the suggestion. However, the presumptive ectoderm
cells often show, in electron micrographs, minute vesicles attached to or
close to the cell membrane, which could be identified with the pinocytotic
structures observed in mammalian tissue cells.
Secondly, the possible mechanism of transmission of the macromolecular component mediating normal induction from the cell of the
organizer to the cell of the reacting ectoderm will be discussed. From
what has been stated in the preceding sections it seems that at the time
of normal induction by the organizer ribonucleoprotein particles may be
transferred to the ectoderm cells. The passage of macromolecules from
the organizer to the reacting ectoderm is also suggested in a quite
different type of experiment by Rounds and Flickinger (1958). When
the ectoderm of the early gastrula of Taricha torosa {Triturus torosus)
was fused to the dorsal mesoderm of the Ranapipiens embryo an increase
in the serological reactivity of the Taricha ectoderm with the anti-serum
against the Rana embryo was noticed. The extracted antigen showed
absorption characteristics typical of nucleoprotein. In a recent paper of
Flickinger, Hatton, and Rounds (1959) the above result was confirmed,
and a histological study of the explant was presented, which indicates
no evidence for migration of Rana cells with small nuclei into the
Taricha ectoderm whose cells are characterized by larger nuclei. No
indication was obtained of the possibility of autoduplication of Rana
macromolecules transferred to the Taricha tissue. Unfortunately the
significance of these interesting experiments in the interpretation of
inductive phenomena is obscured by the fact that in the above experiment the Taricha ectoderm was mostly not induced to form neural
tissue when combined with the Rana organizer. Thus the observed
transfer of macromolecules may have no relation with induction
whatsoever. It is worthwhile to repeat the experiment on a system in
which the frequency of induction is easily controlled, to see whether the
extent of passage of macromolecules is correlated with the frequency of
induction or not. It should be added that discharge of nucleoprotein
from the organizer cells into the culture medium is strongly suggested
in the work of Niu and Twitty (1953) and Niu (1956). Flickinger (1958)
also reported data which may suggest a discharge of macromolecules
having immunological specificity from larval tissues cultured in vitro.
Assuming that a macromolecule is transmitted from the organizer
cell to the ectoderm cell and causes a change in the morphogenesis of the
latter, it is to be expected that any intervention which interferes with
transmission of macromolecules would lead to suppression of induction.
In fact, according to Brächet (1950), insertion of a piece of cellophane
having pores just large enough to permit the passage of nucleotides
TUNEO
YAMADA
also be taken up by the ectoderm cells by pinocytosis. No study has yet
been made to test the suggestion. However, the presumptive ectoderm
cells often show, in electron micrographs, minute vesicles attached to or
close to the cell membrane, which could be identified with the pinocytotic
structures observed in mammalian tissue cells.
Secondly, the possible mechanism of transmission of the macromolecular component mediating normal induction from the cell of the
organizer to the cell of the reacting ectoderm will be discussed. From
what has been stated in the preceding sections it seems that at the time
of normal induction by the organizer ribonucleoprotein particles may be
transferred to the ectoderm cells. The passage of macromolecules from
the organizer to the reacting ectoderm is also suggested in a quite
different type of experiment by Rounds and Flickinger (1958). When
the ectoderm of the early gastrula of Taricha torosa {Triturus torosus)
was fused to the dorsal mesoderm of the Ranapipiens embryo an increase
in the serological reactivity of the Taricha ectoderm with the anti-serum
against the Rana embryo was noticed. The extracted antigen showed
absorption characteristics typical of nucleoprotein. In a recent paper of
Flickinger, Hatton, and Rounds (1959) the above result was confirmed,
and a histological study of the explant was presented, which indicates
no evidence for migration of Rana cells with small nuclei into the
Taricha ectoderm whose cells are characterized by larger nuclei. No
indication was obtained of the possibility of autoduplication of Rana
macromolecules transferred to the Taricha tissue. Unfortunately the
significance of these interesting experiments in the interpretation of
inductive phenomena is obscured by the fact that in the above experiment the Taricha ectoderm was mostly not induced to form neural
tissue when combined with the Rana organizer. Thus the observed
transfer of macromolecules may have no relation with induction
whatsoever. It is worthwhile to repeat the experiment on a system in
which the frequency of induction is easily controlled, to see whether the
extent of passage of macromolecules is correlated with the frequency of
induction or not. It should be added that discharge of nucleoprotein
from the organizer cells into the culture medium is strongly suggested
in the work of Niu and Twitty (1953) and Niu (1956). Flickinger (1958)
also reported data which may suggest a discharge of macromolecules
having immunological specificity from larval tissues cultured in vitro.
Assuming that a macromolecule is transmitted from the organizer
cell to the ectoderm cell and causes a change in the morphogenesis of the
latter, it is to be expected that any intervention which interferes with
transmission of macromolecules would lead to suppression of induction.
In fact, according to Brächet (1950), insertion of a piece of cellophane
having pores just large enough to permit the passage of nucleotides