336
G. V. LOPASHOV
AND O. G.
STROEVA
and of the neural plate at successive developmental stages show that the
eye forming capacity appears during gastrulation, depending on the
approach of the invaginating mesoderm to the eye area. The formation
of the ectomesenchyme, pigment cells, sometimes of ganglia, precedes
the appearance of eyes and parts of the brain. The discrepancies in the
results of different workers seem to be related, first of all, to the size
of the pieces used (large ones in the experiments of Mikami (1941b), and
von Woellwarth (1952); small ones in the experiments of Gallera (1947),
Dragomirova (1949), Adelmann (1930), and Alderman (1935)). In large
pieces of a given developmental stage a higher percentage of eyes
appeared. A rapid increase in the frequency of eye formation takes place
during the course of gastrulation, reaching 100% in von Woellwarth's
experiments (1952) with newts at the stage of formation of the neural
ridge. Before this stage mainly single eyes arise (paired eyes appear only
in the presence of the substrate), while during neurulation the percentage of paired eyes increases, reaching 100% at its termination (von
Woellwarth, 1952). It follows from these experiments that the capacity
to form eyes increases very rapidly within the anterior part of the brain,
but this capacity itself is not yet associated with the segregation of the
eye rudiment into two eyes.
It has already been mentioned that contact of the eye area of the
neural plate with hindbrain mesoderm inhibits eye development. But if
one separates hindbrain areas of the neural plate, at the early neurula
stage, from their substrate and explants them, forebrain structures and
even eyes develop in some cases (Takaya, 1955). After short contact
(from 5 minutes to 5 hours in Ambystoma mexicanum, from 4 to 16 hours
in the newt) of anterior pieces of the chorda rudiment with pieces of
gastrula ectoderm, the most anterior parts of the brain with eyes
appear; only after prolonged contact (over 5 hours in A. mexicanum,
over 16 hours in the newt) do there arise hindbrain structures (Johnen,
1956a, b). Evidently, the cells of the hindbrain pass first through a
phase corresponding to the forebrain rudiment and form the hindbrain
only after a prolonged action of the substrate.
One can understand the significance of the two phases in the induction
of the anterior part of the brain if one considers investigations on the
specificity of inducing substances (Chuang, 1939, 1940; Toivonen, 1940,
1953,1954; Toivonen and Saxén, 1955; Yamada, 1958a, b and others). All
the data available show the presence of two initial substances: a more
stable neurogenic and a less stable mesodermal one, the latter under
certain conditions being transformable to the former (Kawakami and
Mifune, 1957; Yamada, 1958a, 1959). The action of the neurogenic
substance brings about the formation of the most anterior part of the
brain with eyes, while various ratios of these substances lead to the
G. V. LOPASHOV
AND O. G.
STROEVA
and of the neural plate at successive developmental stages show that the
eye forming capacity appears during gastrulation, depending on the
approach of the invaginating mesoderm to the eye area. The formation
of the ectomesenchyme, pigment cells, sometimes of ganglia, precedes
the appearance of eyes and parts of the brain. The discrepancies in the
results of different workers seem to be related, first of all, to the size
of the pieces used (large ones in the experiments of Mikami (1941b), and
von Woellwarth (1952); small ones in the experiments of Gallera (1947),
Dragomirova (1949), Adelmann (1930), and Alderman (1935)). In large
pieces of a given developmental stage a higher percentage of eyes
appeared. A rapid increase in the frequency of eye formation takes place
during the course of gastrulation, reaching 100% in von Woellwarth's
experiments (1952) with newts at the stage of formation of the neural
ridge. Before this stage mainly single eyes arise (paired eyes appear only
in the presence of the substrate), while during neurulation the percentage of paired eyes increases, reaching 100% at its termination (von
Woellwarth, 1952). It follows from these experiments that the capacity
to form eyes increases very rapidly within the anterior part of the brain,
but this capacity itself is not yet associated with the segregation of the
eye rudiment into two eyes.
It has already been mentioned that contact of the eye area of the
neural plate with hindbrain mesoderm inhibits eye development. But if
one separates hindbrain areas of the neural plate, at the early neurula
stage, from their substrate and explants them, forebrain structures and
even eyes develop in some cases (Takaya, 1955). After short contact
(from 5 minutes to 5 hours in Ambystoma mexicanum, from 4 to 16 hours
in the newt) of anterior pieces of the chorda rudiment with pieces of
gastrula ectoderm, the most anterior parts of the brain with eyes
appear; only after prolonged contact (over 5 hours in A. mexicanum,
over 16 hours in the newt) do there arise hindbrain structures (Johnen,
1956a, b). Evidently, the cells of the hindbrain pass first through a
phase corresponding to the forebrain rudiment and form the hindbrain
only after a prolonged action of the substrate.
One can understand the significance of the two phases in the induction
of the anterior part of the brain if one considers investigations on the
specificity of inducing substances (Chuang, 1939, 1940; Toivonen, 1940,
1953,1954; Toivonen and Saxén, 1955; Yamada, 1958a, b and others). All
the data available show the presence of two initial substances: a more
stable neurogenic and a less stable mesodermal one, the latter under
certain conditions being transformable to the former (Kawakami and
Mifune, 1957; Yamada, 1958a, 1959). The action of the neurogenic
substance brings about the formation of the most anterior part of the
brain with eyes, while various ratios of these substances lead to the