15. E M B R Y O G E N E S I S in
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573
activity in amphibian tail after the administration of analogues of
leucine.
(c) Inhibiting action of organic extracts. In the course of the last few years,
several authors have tested the action of organic extracts on embryonic
development. The results of these investigations are in agreement with
the theories of Weiss (1947) and of Rose (1952) who stressed the role of
specific inhibitors in differentiation and growth.
According to Weiss the synthesis of molecules of cytoplasm of a given
organ liberates, on the one hand, "templates" for the production of
new molecules, and on the other hand, diffusible accessory compounds
which are capable of inactivating the former. These latter accumulate
in a common, humoral pool. When they have attained a certain
critical concentration they act as inhibitors and growth ceases. If
partial excision is performed on an organ, the concentration of inhibiting substances is lowered and growth can recommence. Thus can be
explained, on the one hand, the limitation of growth of an organ, and
on the other hand, compensating growth. The requirements of differentiation not being the same as those of growth, organic extracts can
in some cases limit the differentiation of homologous organs, and in
other cases, they can activate growth to the point when the critical
concentration is reached.
According to Rose, inhibiting substances play an important role
in differentiation. When certain tissues or organs have begun their
differentiation under the influence of inducers, the inhibiting substances
which they produce prevent the as yet undifferentiated tissues from developing in the same direction and stimulate their potentialities for
differentiation in other directions.
Investigations have been carried out on different animals in the
course of embryonic development and regeneration, and especially
on the embryo of chick in ovo and in vitro. W e shall consider here only
the experiments done with culture in vitro.
Katoh (1961a) carried out two categories of investigations: experiments on the specific effect of brain extracts incorporated into the
culture medium; and experiments on grafts in cerebral regions.
The experiments involving extracts were done between Hamburger's
stages 4-9. The maximum effect was obtained at stages 4 and 5, i.e. at
the stages of the primitive streak and of head process: it consisted of
deficiencies in the embryonic brain under the influence of brain extracts.
Other organic extracts, such as those of heart, spleen, kidney, mesoderm,
etc. either did not affect the brain at all or had only a very slight effect.
Adult organs have an effect similar to that of the embryonic organs.
In a second experimental series, Katoh united more differentiated
brain, removed at stages 8-11, with whole embryos at stages 4 and 5.
vitfO
573
activity in amphibian tail after the administration of analogues of
leucine.
(c) Inhibiting action of organic extracts. In the course of the last few years,
several authors have tested the action of organic extracts on embryonic
development. The results of these investigations are in agreement with
the theories of Weiss (1947) and of Rose (1952) who stressed the role of
specific inhibitors in differentiation and growth.
According to Weiss the synthesis of molecules of cytoplasm of a given
organ liberates, on the one hand, "templates" for the production of
new molecules, and on the other hand, diffusible accessory compounds
which are capable of inactivating the former. These latter accumulate
in a common, humoral pool. When they have attained a certain
critical concentration they act as inhibitors and growth ceases. If
partial excision is performed on an organ, the concentration of inhibiting substances is lowered and growth can recommence. Thus can be
explained, on the one hand, the limitation of growth of an organ, and
on the other hand, compensating growth. The requirements of differentiation not being the same as those of growth, organic extracts can
in some cases limit the differentiation of homologous organs, and in
other cases, they can activate growth to the point when the critical
concentration is reached.
According to Rose, inhibiting substances play an important role
in differentiation. When certain tissues or organs have begun their
differentiation under the influence of inducers, the inhibiting substances
which they produce prevent the as yet undifferentiated tissues from developing in the same direction and stimulate their potentialities for
differentiation in other directions.
Investigations have been carried out on different animals in the
course of embryonic development and regeneration, and especially
on the embryo of chick in ovo and in vitro. W e shall consider here only
the experiments done with culture in vitro.
Katoh (1961a) carried out two categories of investigations: experiments on the specific effect of brain extracts incorporated into the
culture medium; and experiments on grafts in cerebral regions.
The experiments involving extracts were done between Hamburger's
stages 4-9. The maximum effect was obtained at stages 4 and 5, i.e. at
the stages of the primitive streak and of head process: it consisted of
deficiencies in the embryonic brain under the influence of brain extracts.
Other organic extracts, such as those of heart, spleen, kidney, mesoderm,
etc. either did not affect the brain at all or had only a very slight effect.
Adult organs have an effect similar to that of the embryonic organs.
In a second experimental series, Katoh united more differentiated
brain, removed at stages 8-11, with whole embryos at stages 4 and 5.