12
TUNEO
YAMADA
interest also is the fact that the outer nuclear membrane which is at
first free of particulate components becomes encrusted with the fine
particles at the tail-bud stage when also the endoplasmic reticulum
becomes rough-surfaced.
I. Summary
Summarizing the electron microscopic study of the fine structure of
the embryonic cells in the Amphibia, it may be stated that the basic
organelles of the tissue cells are already present before cellular differentiation, beside yolk platelets which are characteristic of embryonic cells.
But there occur progressive changes in some of the basic organelles
which result in their elaboration. Further it may be added that the
cell-types of different embryological value are distinguishable only by
quantitative differences of some organelles or by a temporal shift of
some morphological changes. The similarity of different cell-types is
much more impressive than their divergence. This is, however, not to
be wondered at, since even in the so-called mosaic eggs the difference in
fine structure of blastomeres of various developmental abilities is
quantitative rather than qualitative (Lehmann, 1958). On the other
hand, morphological changes progressing with development have been
noticed, which may be of importance for elucidating the mechanism of
the organizer action and its consequences.
III. Dorsalization by Subcytolytic Treatments
By introducing inorganic substances such as fuller's earth, silica, and
calcium carbonate into the blastocoel of the young Triturus gastrula,
Yo. K. Okada (1938) obtained neural induction. The result was interpreted as a relay mechanism: the cells disintegrated by implantation of
a foreign substance were assumed to be responsible for providing the
stimulus for the formation of the secondary neural plate. That a piece
of the ectoderm separated from the early gastrula and cultured in a salt
solution can undergo spontaneous neuralization was first shown by
Barth (1941). Holtfreter (1944, 1945, 1947) took up this important
problem, and reached the conclusion that transient cell injuries are
responsible for the neuralization. The ectoderm of Ambystoma punctatum, which was used in the original experiment of Barth, is very
sensitive to exposure to Holtfreter solution, discharges a large number
of free cells, and is frequently neuralized, while the ectoderm of Triturus
torosus develops under the same condition without a sign of disintegration and gives rise to an irregular cluster of epidermal cells. Mutual
adhesion of embryonic cells depends upon the pH and the presence of
TUNEO
YAMADA
interest also is the fact that the outer nuclear membrane which is at
first free of particulate components becomes encrusted with the fine
particles at the tail-bud stage when also the endoplasmic reticulum
becomes rough-surfaced.
I. Summary
Summarizing the electron microscopic study of the fine structure of
the embryonic cells in the Amphibia, it may be stated that the basic
organelles of the tissue cells are already present before cellular differentiation, beside yolk platelets which are characteristic of embryonic cells.
But there occur progressive changes in some of the basic organelles
which result in their elaboration. Further it may be added that the
cell-types of different embryological value are distinguishable only by
quantitative differences of some organelles or by a temporal shift of
some morphological changes. The similarity of different cell-types is
much more impressive than their divergence. This is, however, not to
be wondered at, since even in the so-called mosaic eggs the difference in
fine structure of blastomeres of various developmental abilities is
quantitative rather than qualitative (Lehmann, 1958). On the other
hand, morphological changes progressing with development have been
noticed, which may be of importance for elucidating the mechanism of
the organizer action and its consequences.
III. Dorsalization by Subcytolytic Treatments
By introducing inorganic substances such as fuller's earth, silica, and
calcium carbonate into the blastocoel of the young Triturus gastrula,
Yo. K. Okada (1938) obtained neural induction. The result was interpreted as a relay mechanism: the cells disintegrated by implantation of
a foreign substance were assumed to be responsible for providing the
stimulus for the formation of the secondary neural plate. That a piece
of the ectoderm separated from the early gastrula and cultured in a salt
solution can undergo spontaneous neuralization was first shown by
Barth (1941). Holtfreter (1944, 1945, 1947) took up this important
problem, and reached the conclusion that transient cell injuries are
responsible for the neuralization. The ectoderm of Ambystoma punctatum, which was used in the original experiment of Barth, is very
sensitive to exposure to Holtfreter solution, discharges a large number
of free cells, and is frequently neuralized, while the ectoderm of Triturus
torosus develops under the same condition without a sign of disintegration and gives rise to an irregular cluster of epidermal cells. Mutual
adhesion of embryonic cells depends upon the pH and the presence of