546
E T I E N N E W O L F F
posterior region of the head; and that other organs, such as the kidney
of mice, and the liver of perch, viper and guinea-pig, induce almost
exclusively anterior brain.
We owe to Toivonen (1950) a well-designed investigation into the
properties of artificial inducers and the initiation of research into their
chemical nature. He worked, in particular, with two artificial inducers
whose properties are quite different: guinea-pig liver, which is basically
an archencephalic inducer; and guinea-pig kidney, basically a spinal
inducer. Archencephalic inducing substances, extracted from guineapig liver, are heat-stable, soluble in organic liquids, and dialysable;
in the course of fractionation, they accompany fatty acids and nucleoproteins. Spinal inducing substances, extracted from guinea-pig
kidney, are heat-labile, and insoluble in petroleum ether; they do not
accompany nucleoproteins. These results orientated research towards
the solution which prevails today.
Another important result obtained by Toivonen (1953a, b, 1954)
was the discovery of a purely "mesodermizing" inducer. An extract of
guinea-pig bone marrow, treated with alcohol at 90°C, induces almost
exclusively mesodermal organs of the trunk and the tail-mesenchyme,
melanophores, pronephros, limb-buds and fins, myotomes, and notochord. It should be noted further that, in contrast to guinea-pig kidney,
this extract does not induce differentiation of spinal cord. Guinea-pig
kidney, therefore, could be considered to be a more complex inducer.
Toivonen wondered what would be the result of the combined
action of a mesodermal inducer (extract of bone marrow) and an archencephalic inducer (guinea-pig liver). Experiments involving implantation
into a gastrula and explantation in an ectodermal envelope produced
embryonic formations which possessed most of the differentiations of a
normal embryo. Many formations, which did not appear in the controls, where only one of the two inducers was implanted, were present in
experiments where the two inducers were associated. More particularly, he observed the presence of a spinal-cord, and of posterior
brain with otocysts. The whole organized itself into a more or less
harmonious system which gave the impression of a real embryo,
particularly in the trunco-caudal region.
The deuterencephalic and the spino-caudal formations can be
interpreted as results of the interactions between the activities of the
two inducers. T w o gradients of influence would become manifest in the
longitudinal plane as well as in the transverse one: one emanated from
a neuralizing substance, jV, which, in its pure state in the proximal
median region, would induce the archencephalon; the other was
caused by the mesodermal substance, M, localized in the distal region.
This last (M) would combine in varying proportions with the nueralizing
E T I E N N E W O L F F
posterior region of the head; and that other organs, such as the kidney
of mice, and the liver of perch, viper and guinea-pig, induce almost
exclusively anterior brain.
We owe to Toivonen (1950) a well-designed investigation into the
properties of artificial inducers and the initiation of research into their
chemical nature. He worked, in particular, with two artificial inducers
whose properties are quite different: guinea-pig liver, which is basically
an archencephalic inducer; and guinea-pig kidney, basically a spinal
inducer. Archencephalic inducing substances, extracted from guineapig liver, are heat-stable, soluble in organic liquids, and dialysable;
in the course of fractionation, they accompany fatty acids and nucleoproteins. Spinal inducing substances, extracted from guinea-pig
kidney, are heat-labile, and insoluble in petroleum ether; they do not
accompany nucleoproteins. These results orientated research towards
the solution which prevails today.
Another important result obtained by Toivonen (1953a, b, 1954)
was the discovery of a purely "mesodermizing" inducer. An extract of
guinea-pig bone marrow, treated with alcohol at 90°C, induces almost
exclusively mesodermal organs of the trunk and the tail-mesenchyme,
melanophores, pronephros, limb-buds and fins, myotomes, and notochord. It should be noted further that, in contrast to guinea-pig kidney,
this extract does not induce differentiation of spinal cord. Guinea-pig
kidney, therefore, could be considered to be a more complex inducer.
Toivonen wondered what would be the result of the combined
action of a mesodermal inducer (extract of bone marrow) and an archencephalic inducer (guinea-pig liver). Experiments involving implantation
into a gastrula and explantation in an ectodermal envelope produced
embryonic formations which possessed most of the differentiations of a
normal embryo. Many formations, which did not appear in the controls, where only one of the two inducers was implanted, were present in
experiments where the two inducers were associated. More particularly, he observed the presence of a spinal-cord, and of posterior
brain with otocysts. The whole organized itself into a more or less
harmonious system which gave the impression of a real embryo,
particularly in the trunco-caudal region.
The deuterencephalic and the spino-caudal formations can be
interpreted as results of the interactions between the activities of the
two inducers. T w o gradients of influence would become manifest in the
longitudinal plane as well as in the transverse one: one emanated from
a neuralizing substance, jV, which, in its pure state in the proximal
median region, would induce the archencephalon; the other was
caused by the mesodermal substance, M, localized in the distal region.
This last (M) would combine in varying proportions with the nueralizing