350
G. V. LOPASHOV AND
O. G.
STROEVA
embryos at either neurula or tail-bud stages (Liedke, 1942—Rana
pipiens, 1951, 1955—Ambystoma punctatum; Reyer, 1958a, b—
Triturus viridiscens and Ambystoma punctatum; Jacobson, 1958—
Taricha torosa). These experiments revealed that lenses arose from the
belly ectoderm only after transplantation from late gastrula to neurula,
while the ectoderm of younger gastrulae did not form lenses. The
neurula ectoderm formed them only in the experiments with newts. On
the contrary, the head ectoderm often formed lenses from the middle
gastrula stage, when the mesodermal substrate came in contact with it
(in particular, with the presumptive lens-forming area). Apart from the
justifiable conclusion as to the importance of the preceding action of
entomesoderm for lens induction, leading to a greater readiness to
react to the subsequent action of the eye, the authors mentioned draw
the conclusion that the eye cannot induce lens formation without this
influence. This conclusion is based mainly on the fact that lenses have
not arisen from early gastrula ectoderm. However, there are many data
which contradict this (Schmidt and Ragosina, 1937; Lopashov, 1937;
Woerdeman, 1938; Sheina, 1940, 1944; Perri, 1934; Brahma, 1959 and
others): the eye rudiment alone brought into contact with gastrula,
neurula and tail-bud stage ectoderm has induced lens formation. In all
these experiments the ectoderm was brought into contact with the eye
only, which induces in the ectoderm of the gastrula nothing but lenses.
In the experiments involving transplantations to the head, the ectoderm was exposed to the action of other parts surrounding the eye, and
especially of the brain. This led to the formation of multiple brain
rudiments, which could hinder further induction of lenses, in particular
owing to the simultaneous origin of large amounts of ectomesenchyme
preventing ectodermal contact with the eye.
It would be more reasonable to suggest that the true significance of
these two phases of lens induction consists in the following: (1) both the
entomesoderm alone, and, in particular, the eye alone can induce lens
formation in the ectoderm, depending on the stage of the latter; (2) the
relative significance of these two agents differs in various species in
relation to the rate of ectoderm ageing. In some species the primary
action of entomesoderm is required to a greater extent for the subsequent action of the eye in normal development than in other species;
(3) previous participation in the whole complex of movements proceeding in the embryo head, contributes to the provision of a close contact
between the ectoderm and the eye rudiment; with the increase of the
closeness of this contact the frequency of lens induction is strongly
increased (Lopashov, 1960). This picture is similar to that found for the
ear vesicle (Ginsburg, 1946, 1950; Yntema, 1950, 1955).
The primary effect differs from the action of the eye in several
G. V. LOPASHOV AND
O. G.
STROEVA
embryos at either neurula or tail-bud stages (Liedke, 1942—Rana
pipiens, 1951, 1955—Ambystoma punctatum; Reyer, 1958a, b—
Triturus viridiscens and Ambystoma punctatum; Jacobson, 1958—
Taricha torosa). These experiments revealed that lenses arose from the
belly ectoderm only after transplantation from late gastrula to neurula,
while the ectoderm of younger gastrulae did not form lenses. The
neurula ectoderm formed them only in the experiments with newts. On
the contrary, the head ectoderm often formed lenses from the middle
gastrula stage, when the mesodermal substrate came in contact with it
(in particular, with the presumptive lens-forming area). Apart from the
justifiable conclusion as to the importance of the preceding action of
entomesoderm for lens induction, leading to a greater readiness to
react to the subsequent action of the eye, the authors mentioned draw
the conclusion that the eye cannot induce lens formation without this
influence. This conclusion is based mainly on the fact that lenses have
not arisen from early gastrula ectoderm. However, there are many data
which contradict this (Schmidt and Ragosina, 1937; Lopashov, 1937;
Woerdeman, 1938; Sheina, 1940, 1944; Perri, 1934; Brahma, 1959 and
others): the eye rudiment alone brought into contact with gastrula,
neurula and tail-bud stage ectoderm has induced lens formation. In all
these experiments the ectoderm was brought into contact with the eye
only, which induces in the ectoderm of the gastrula nothing but lenses.
In the experiments involving transplantations to the head, the ectoderm was exposed to the action of other parts surrounding the eye, and
especially of the brain. This led to the formation of multiple brain
rudiments, which could hinder further induction of lenses, in particular
owing to the simultaneous origin of large amounts of ectomesenchyme
preventing ectodermal contact with the eye.
It would be more reasonable to suggest that the true significance of
these two phases of lens induction consists in the following: (1) both the
entomesoderm alone, and, in particular, the eye alone can induce lens
formation in the ectoderm, depending on the stage of the latter; (2) the
relative significance of these two agents differs in various species in
relation to the rate of ectoderm ageing. In some species the primary
action of entomesoderm is required to a greater extent for the subsequent action of the eye in normal development than in other species;
(3) previous participation in the whole complex of movements proceeding in the embryo head, contributes to the provision of a close contact
between the ectoderm and the eye rudiment; with the increase of the
closeness of this contact the frequency of lens induction is strongly
increased (Lopashov, 1960). This picture is similar to that found for the
ear vesicle (Ginsburg, 1946, 1950; Yntema, 1950, 1955).
The primary effect differs from the action of the eye in several