544
E T I E N N E
W O L F F
variety of treatments. It is essentially in the responsive tissue that one
must search for the factors of differentiation.
It is necessary to point out, however, that in neuralization without
an inducer, neural differentiations are in general anarchic, and uncoordinated; when they are orderly, they are of the archencephalic
type which is the commonest form of induction obtainable by most of
the methods. They do not cause development of either deuterencephalic or of spinocaudal parts of the nervous system, nor do they produce any mesodermal differentiation. One may conclude, therefore,
that non-specific treatments cannot take the place of natural inducers.
It is thanks to the discovery of heterogeneous inducers with regional
influence that the problem has been recently elucidated, as we shall see
further on (p. 549-53).
4. The Mode of Action of Inducing Agents
Since numerous tissues or extracts have an inducing effect on presumptive ectoderm explanted in vitro, it seems natural to assume that
chemical reactions occur between the inducer and the reactor. Can
one isolate diffusible inducing substances, acting from a distance, or is
inducing action possible only if there is close contact between the inducer
and the responsive tissue? Both points of view have had support.
(a) Many authors postulate the existence of a diffusible substance
which can be separated from the inducer. The experiments of Niu and
Twitty (1953) have produced data favourable to this hypothesis. These
authors cultured, for 8 days, organizer tissues, i.e. blastoporal lips,
posterior neural plates, somitic blocks, in a medium differing only
slightly from Holtfreter's solution. Then pieces of indifferent ectoderm
were placed in the same medium, at a distance from the first explants.
These dissociated, in such a milieu, into isolated cells which would then
become transformed into ganglionic-type cells, i.e. chromatophores or
neural cells or myoblasts. The same results have been obtained in a
medium from which the organizer tissues were removed before the
ectoderm was placed in the culture. No differentiation could, however,
be obtained in a liquid in which non-inducing tissues, such as the ectoderm or endoderm, had been previously cultured.
The investigations of Von Woellwarth (1956) led to analogous results.
He used fluid obtained from centrifuged extract of chick embryo and
diluted with Holtfreter's solution. Fragments of ectoderm, plunged
into this medium, rolled themselves into vesicles with the surface
lacking the pellicle (Holtfreter "coat") pointing outwards. Such an
arrangement is of a type to favour inducing action. The explants
remained coherent. All contained pigment cells and mesenchyme. Some
showed cephalic or caudal differentiations. The neural tissues were
E T I E N N E
W O L F F
variety of treatments. It is essentially in the responsive tissue that one
must search for the factors of differentiation.
It is necessary to point out, however, that in neuralization without
an inducer, neural differentiations are in general anarchic, and uncoordinated; when they are orderly, they are of the archencephalic
type which is the commonest form of induction obtainable by most of
the methods. They do not cause development of either deuterencephalic or of spinocaudal parts of the nervous system, nor do they produce any mesodermal differentiation. One may conclude, therefore,
that non-specific treatments cannot take the place of natural inducers.
It is thanks to the discovery of heterogeneous inducers with regional
influence that the problem has been recently elucidated, as we shall see
further on (p. 549-53).
4. The Mode of Action of Inducing Agents
Since numerous tissues or extracts have an inducing effect on presumptive ectoderm explanted in vitro, it seems natural to assume that
chemical reactions occur between the inducer and the reactor. Can
one isolate diffusible inducing substances, acting from a distance, or is
inducing action possible only if there is close contact between the inducer
and the responsive tissue? Both points of view have had support.
(a) Many authors postulate the existence of a diffusible substance
which can be separated from the inducer. The experiments of Niu and
Twitty (1953) have produced data favourable to this hypothesis. These
authors cultured, for 8 days, organizer tissues, i.e. blastoporal lips,
posterior neural plates, somitic blocks, in a medium differing only
slightly from Holtfreter's solution. Then pieces of indifferent ectoderm
were placed in the same medium, at a distance from the first explants.
These dissociated, in such a milieu, into isolated cells which would then
become transformed into ganglionic-type cells, i.e. chromatophores or
neural cells or myoblasts. The same results have been obtained in a
medium from which the organizer tissues were removed before the
ectoderm was placed in the culture. No differentiation could, however,
be obtained in a liquid in which non-inducing tissues, such as the ectoderm or endoderm, had been previously cultured.
The investigations of Von Woellwarth (1956) led to analogous results.
He used fluid obtained from centrifuged extract of chick embryo and
diluted with Holtfreter's solution. Fragments of ectoderm, plunged
into this medium, rolled themselves into vesicles with the surface
lacking the pellicle (Holtfreter "coat") pointing outwards. Such an
arrangement is of a type to favour inducing action. The explants
remained coherent. All contained pigment cells and mesenchyme. Some
showed cephalic or caudal differentiations. The neural tissues were