15. E M B R Y O GENESIS in vitro
543
Amongst the inactive substances, there are few animal tissues but
many vegetable extracts and inert substances.
The astonishing wealth of heterogeneous inducing agents in the animal world allowed one to hope that biochemical research on the nature
of active substances would yield quick results. In fact, the problem
turned out to be rather more complicated. For many years, attempts at
extraction of inducing tissues and the testing of various chemical
substances gave uncertain and contradictory results. Different hypotheses were proposed one after another: there have been theories
involving glycogen (Spemann, Fischer and Wehmeier, 1933), sterols
(Waddington, Needham and Nowinski, 1936), acids (Fischer, Wehmeier, Lehmann, Juhling and Hultzsch, 1935), ribonucleoproteins
(Brachet, 1944), and proteins (Kuusi, 1951). Certain authors, such as
Barth and Graff (1938), even came to think that any action whatsoever
by a toxic substance, such as cephalin, digitonin, or the acidified white
of an egg, was likely to provoke neuralization of the ectoderm. Other
substances, such as methylene blue (Waddington, Needham and
Brachet, 1936), sulphydryl compounds (Brachet and Rapkine, 1939),
and ammonia (Yamada, 1950a) show analogous effects.
The last hypothesis was backed up by the interesting discovery, due
to Barth (1941), of neuralization without an inducer. The ectoderm
of the gastrula of the salamander, Amblystoma punctatum, explanted
by itself into Holtfreter's solution, transformed itself spontaneously into
nervous tissue. This result was confirmed by Holtfreter (1944) on this
same species. Nevertheless, it does not hold good for the ectoderm of
other species of Urodeles, such as Triturus torosus, which, in the same
solution, does not differentiate any neural tissue whatever. However, it
is possible to suppress the neuralizing effect obtained with Amblystoma
punctatum (Holtfreter, 1945) by modifying the standard medium which
is known to be toxic to the ectoderm of this species. Such is the effect
of a slight acidification ofthe medium, or ofthe addition of "protective"
substances such as glucose, sucrose, and histone. Conversely, one can
obtain neuralization in Triturus torosus without an inducer (Holtfreter,
1947) by modifying the standard medium which normally suits this
species. One simply adds to the medium a little distilled water or alcohol,
or alters the pH. Holtfreter shows that in all cases of neuralization
without an inducer, certain damage occurs to the cells of the ectoderm
which he describes as precytolysis. These lesions are reversible, but
their effect is either to modify cellular permeability, or to provoke
syntheses of new substances in the actual cytoplasm of the responsive
tissue. It might be concluded that these combinations of effects are the
same as those which are produced by the natural inducer. It is very
tempting to consider the neuralizing action as a common response to a
543
Amongst the inactive substances, there are few animal tissues but
many vegetable extracts and inert substances.
The astonishing wealth of heterogeneous inducing agents in the animal world allowed one to hope that biochemical research on the nature
of active substances would yield quick results. In fact, the problem
turned out to be rather more complicated. For many years, attempts at
extraction of inducing tissues and the testing of various chemical
substances gave uncertain and contradictory results. Different hypotheses were proposed one after another: there have been theories
involving glycogen (Spemann, Fischer and Wehmeier, 1933), sterols
(Waddington, Needham and Nowinski, 1936), acids (Fischer, Wehmeier, Lehmann, Juhling and Hultzsch, 1935), ribonucleoproteins
(Brachet, 1944), and proteins (Kuusi, 1951). Certain authors, such as
Barth and Graff (1938), even came to think that any action whatsoever
by a toxic substance, such as cephalin, digitonin, or the acidified white
of an egg, was likely to provoke neuralization of the ectoderm. Other
substances, such as methylene blue (Waddington, Needham and
Brachet, 1936), sulphydryl compounds (Brachet and Rapkine, 1939),
and ammonia (Yamada, 1950a) show analogous effects.
The last hypothesis was backed up by the interesting discovery, due
to Barth (1941), of neuralization without an inducer. The ectoderm
of the gastrula of the salamander, Amblystoma punctatum, explanted
by itself into Holtfreter's solution, transformed itself spontaneously into
nervous tissue. This result was confirmed by Holtfreter (1944) on this
same species. Nevertheless, it does not hold good for the ectoderm of
other species of Urodeles, such as Triturus torosus, which, in the same
solution, does not differentiate any neural tissue whatever. However, it
is possible to suppress the neuralizing effect obtained with Amblystoma
punctatum (Holtfreter, 1945) by modifying the standard medium which
is known to be toxic to the ectoderm of this species. Such is the effect
of a slight acidification ofthe medium, or ofthe addition of "protective"
substances such as glucose, sucrose, and histone. Conversely, one can
obtain neuralization in Triturus torosus without an inducer (Holtfreter,
1947) by modifying the standard medium which normally suits this
species. One simply adds to the medium a little distilled water or alcohol,
or alters the pH. Holtfreter shows that in all cases of neuralization
without an inducer, certain damage occurs to the cells of the ectoderm
which he describes as precytolysis. These lesions are reversible, but
their effect is either to modify cellular permeability, or to provoke
syntheses of new substances in the actual cytoplasm of the responsive
tissue. It might be concluded that these combinations of effects are the
same as those which are produced by the natural inducer. It is very
tempting to consider the neuralizing action as a common response to a