15. E M B R Y O G E N E S I S in vitro
551
explants of the ventral mesoderm of Triturus pyrrhogaster. These explants,
normally, in vitro, differentiate into blood-islands, mesothelium, and
nephric tubes. In the presence of undifferentiated ectoderm, they induce
only occasional melanophores in it. When treated with sodium thiocyanate, sodium iodide, urea, ammonia, hydrochloric acid or zinc
chloride, they become "dorsalized" and differentiate into mesodermal
formations such as the notochord and well metamerized somites,
probably of a caudal nature. In this case, blood-islands still appear but
are in reduced quantity. What differentiations can these explants
induce in an ectodermal "sandwich"? Whatever be the treatment
undergone, neural formations are induced. They are generally deuterencephalic and spino-caudal structures in the case of the first four agents,
but in the case of hydrochloric acid and zinc chloride they are archencephalic or anterior structures (eyes, lenses, nose, and anterior limbs).
The results of this experiment tend to prove, in the first place, that
a chemical substance can modify the prospective value of a ventral
mesodermal area, and thus can give rise to an inducing activity that
the area does not otherwise possess; in the second place, that extracts
thus dorsalized induce neural structures. These last are spino-caudal
and deuterencephalic when the chemical agent is a relatively mild one.
Treatment with hydrochloric acid and with zinc chloride, which may
be considered to be more toxic, gives rise to archencephalic inductions.
T o conclude, we can say that the inducing substances of the mesoderm and of the spinal and deuterencephalic regions of the nervous
system prove to be much more fragile to thermal treatment and to
various chemical treatments than the inducing substances of the archencephalon. When the mesodermal or spino-caudal inducers are inactivated, they become transformed into archencephalic inducers
(Yamada, 1959) or they unmask the substances responsible for this
inducing action (Toivonen, 1950, 1954).
2. Enzymic Actions
Many investigations have pointed to the idea that two categories of
extracts or of substances may be considered as inducing agents: protein
fractions and extracts rich in ribonucleoproteins. The first of these, in
particular, were prepared starting from spino-caudal and deuterencephalic inducers of guinea-pig kidney, and from mesodermic inducers
of the extract of bone marrow or of other tissues such as the muscle
of beef. The second of these, extracted more particularly from guineapig liver, possess an archencephalic activity.
It has already been noted that pentose-ribonucleoproteins do not owe
their activity to their ribonucleic acids but rather to the associated
protein. Several authors have shown that they are affected neither by
551
explants of the ventral mesoderm of Triturus pyrrhogaster. These explants,
normally, in vitro, differentiate into blood-islands, mesothelium, and
nephric tubes. In the presence of undifferentiated ectoderm, they induce
only occasional melanophores in it. When treated with sodium thiocyanate, sodium iodide, urea, ammonia, hydrochloric acid or zinc
chloride, they become "dorsalized" and differentiate into mesodermal
formations such as the notochord and well metamerized somites,
probably of a caudal nature. In this case, blood-islands still appear but
are in reduced quantity. What differentiations can these explants
induce in an ectodermal "sandwich"? Whatever be the treatment
undergone, neural formations are induced. They are generally deuterencephalic and spino-caudal structures in the case of the first four agents,
but in the case of hydrochloric acid and zinc chloride they are archencephalic or anterior structures (eyes, lenses, nose, and anterior limbs).
The results of this experiment tend to prove, in the first place, that
a chemical substance can modify the prospective value of a ventral
mesodermal area, and thus can give rise to an inducing activity that
the area does not otherwise possess; in the second place, that extracts
thus dorsalized induce neural structures. These last are spino-caudal
and deuterencephalic when the chemical agent is a relatively mild one.
Treatment with hydrochloric acid and with zinc chloride, which may
be considered to be more toxic, gives rise to archencephalic inductions.
T o conclude, we can say that the inducing substances of the mesoderm and of the spinal and deuterencephalic regions of the nervous
system prove to be much more fragile to thermal treatment and to
various chemical treatments than the inducing substances of the archencephalon. When the mesodermal or spino-caudal inducers are inactivated, they become transformed into archencephalic inducers
(Yamada, 1959) or they unmask the substances responsible for this
inducing action (Toivonen, 1950, 1954).
2. Enzymic Actions
Many investigations have pointed to the idea that two categories of
extracts or of substances may be considered as inducing agents: protein
fractions and extracts rich in ribonucleoproteins. The first of these, in
particular, were prepared starting from spino-caudal and deuterencephalic inducers of guinea-pig kidney, and from mesodermic inducers
of the extract of bone marrow or of other tissues such as the muscle
of beef. The second of these, extracted more particularly from guineapig liver, possess an archencephalic activity.
It has already been noted that pentose-ribonucleoproteins do not owe
their activity to their ribonucleic acids but rather to the associated
protein. Several authors have shown that they are affected neither by