I. PROBLEM OF THE ORGANIZER
13
calcium ions. When the torosus ectoderm is put in a medium of extremely
low or high pH (below 5-0 and above 9-2), lacking calcium ions, containing 10% ethanol, or which is hypotonic, it disintegrates into individual cells. When the disintegrating explant is transferred to the
standard culture medium, cells reaggregate in a compact cell mass.
Upon further culture in the standard medium, the reaggregate very
often develops neural tissues including archencephalic structures and
mesenchyme (Holtfreter, 1945, 1947). The possibility that the cells
killed by the treatment acted on the healthy cells to produce the neural
structure (the relay mechanism), was excluded by a series of experiments
(Holtfreter, 1947), in which dying cells were removed by shaking, and
still neural differentiation of the remaining cell aggregate was observed.
Consequently, the treatment established within the cells a condition
which leads to neural differentiation of the cells themselves. Holtfreter
(1947) argued that 'an autolytic dissociation of some protoplasmic
Compounds' results in the release of 'the neuralizing agent (evocator)
proper', and he identified the mechanism of the phenomenon as the
acquisition of ability to perform neural induction by an embryonic area
killed by chemical and physical means (Section V).
These experiments of Holtfreter on the ectoderm of Triturus torosus
were confirmed in their main features in a series of experiments conducted
on Triturus pyrrhogaster in our laboratory (Yamada, 1950a; Karasaki,
1957a, b). However, in contrast to Holtfreter's results, disaggregation
of the ectoderm in Holtfreter's solution without calcium ions or containing 15% ethanol was found not to be followed by neuralization
(Karasaki, 1957a). A similar disaggregating treatment of the ectoderm
in Holtfreter's solution containing M/8 sucrose or M/16 lithium chloride
also failed to give neuralization. Thus disaggregation of the ectoderm
does not constitute a sufficient condition for neuralization. In any case,
pyrrhogaster ectoderm seems to be less sensitive to this type of treatment
than torosus ectoderm. With the hope of elucidating the mechanism
whereby neuralization (dorsalization) is brought about within the cell,
Karasaki (1957a) made a study, using the phase contrast microscope,
of ectoderm cells disaggregated with ammonia, sodium hydroxide,
hydrochloric acid, urea, and sodium thiocyanate which all cause
neuralization. He compared his findings with those obtained on cells
disaggregated with calcium-free solution, sucrose, and lithium chloride
solutions which do not cause neuralization. Solation of the endoplasm
of the treated cells accompanied by disappearance of the endoplasmic
gel layer, and release of mucous substance were observable after the
treatment with the first group of agents, but hardly noticeable after the
treatment with the second group of agents. Further, suppression of
neuralization evoked by ammonia treatment was found to be possible
13
calcium ions. When the torosus ectoderm is put in a medium of extremely
low or high pH (below 5-0 and above 9-2), lacking calcium ions, containing 10% ethanol, or which is hypotonic, it disintegrates into individual cells. When the disintegrating explant is transferred to the
standard culture medium, cells reaggregate in a compact cell mass.
Upon further culture in the standard medium, the reaggregate very
often develops neural tissues including archencephalic structures and
mesenchyme (Holtfreter, 1945, 1947). The possibility that the cells
killed by the treatment acted on the healthy cells to produce the neural
structure (the relay mechanism), was excluded by a series of experiments
(Holtfreter, 1947), in which dying cells were removed by shaking, and
still neural differentiation of the remaining cell aggregate was observed.
Consequently, the treatment established within the cells a condition
which leads to neural differentiation of the cells themselves. Holtfreter
(1947) argued that 'an autolytic dissociation of some protoplasmic
Compounds' results in the release of 'the neuralizing agent (evocator)
proper', and he identified the mechanism of the phenomenon as the
acquisition of ability to perform neural induction by an embryonic area
killed by chemical and physical means (Section V).
These experiments of Holtfreter on the ectoderm of Triturus torosus
were confirmed in their main features in a series of experiments conducted
on Triturus pyrrhogaster in our laboratory (Yamada, 1950a; Karasaki,
1957a, b). However, in contrast to Holtfreter's results, disaggregation
of the ectoderm in Holtfreter's solution without calcium ions or containing 15% ethanol was found not to be followed by neuralization
(Karasaki, 1957a). A similar disaggregating treatment of the ectoderm
in Holtfreter's solution containing M/8 sucrose or M/16 lithium chloride
also failed to give neuralization. Thus disaggregation of the ectoderm
does not constitute a sufficient condition for neuralization. In any case,
pyrrhogaster ectoderm seems to be less sensitive to this type of treatment
than torosus ectoderm. With the hope of elucidating the mechanism
whereby neuralization (dorsalization) is brought about within the cell,
Karasaki (1957a) made a study, using the phase contrast microscope,
of ectoderm cells disaggregated with ammonia, sodium hydroxide,
hydrochloric acid, urea, and sodium thiocyanate which all cause
neuralization. He compared his findings with those obtained on cells
disaggregated with calcium-free solution, sucrose, and lithium chloride
solutions which do not cause neuralization. Solation of the endoplasm
of the treated cells accompanied by disappearance of the endoplasmic
gel layer, and release of mucous substance were observable after the
treatment with the first group of agents, but hardly noticeable after the
treatment with the second group of agents. Further, suppression of
neuralization evoked by ammonia treatment was found to be possible