342
G. V. LOPASHOV AND O. G.
STROEVA
arising within the retina (Lopashov, 1951, 1960). This shows the similarity of early processes in both of the eye layers. However, upon their
further divergent development, cytoplasmic components belonging to
the other type of differentiation are eliminated from the internal layer.
In all the animals investigated (Dragomirov, 1935, on newts;
Lopashov, 1960, on various Amphibians; Dabaghian, 1959, on Acipenserid fishes) the possibility of the formation of pigment epithelium from
the internal layer becomes considerably reduced with age, and ceases at
the stage preceding the differentiation of the retina into layers. On the
other hand, though the capacity of the external layer to transform into
retina disappears before its pigmentation in Birds (Alexander, 1937;
Dorris, 1938; Reinbold, 1958), in Acipenserid fishes (Dabaghian, 1959)
and Mammals (Stroeva, 1960) it is preserved for a short time after the
onset of pigmentation of the external layer. In Anurans the pigment epithelium will transform into retina up to the time of metamorphosis
(Lopashov, 1949, 1959; Sato, 1953), while in Urodeles (Wachs, 1920;
Ikeda, 1935; Stone, 1950a, b; Stone and Steinitz, 1957; Reyer, 1956;
Hasegawa, 1958) the power is preserved throughout their whole life.
The regeneration of the retina in Urodeles, and that of the retina and the
lens in newts and salamanders, is based on such metaplasia (Reyer,
1954b).
The proportions of the pigment epithelium and of the retina are not
primarily determined in the eye rudiments. They depend on the ratio of
the internal tension, produced by the fluid which at this stage fills the
cavity common to the brain and eye vesicles, and the pressure of the
parts surrounding the eye. When space is restricted, most of the eye
rudiment is forced into aggregation. This portion differentiates into
retina, localized at the site of contact with the lens. The surface layer of
the eye rudiment, which borders the mesenchyme, develops into pigment epithelium (Lopashov, 1960).
This mechanism of polarization of the eye rudiment does not provide
for the emergence of a regular cup-shaped form of the eye. It leads to
the formation of an irregular aggregation when eye rudiments are
cultivated without the lens either in a saline solution, or in body cavity
fluids (Dorris, 1938; Lopashov, 1945, 1960; Dabaghian, 1958,1959); and
similarly no complete invagination of the eye rudiment is observed in
whole embryos devoid of lenses, with an incomplete contact with the
ectoderm. It also leads to the formation of folds in the retina, to the
evagination of the bottom of the cup towards the ectoderm, and to
inhibition of the development of the iris, anterior chamber and vitreous
chamber (Lehmann, 1934, 1936; Reyer, 1948, 1950). The close adhesion
of the eye vesicle to the lens material, which takes place in normal development, not only contributes to the origin of the retina here, but
G. V. LOPASHOV AND O. G.
STROEVA
arising within the retina (Lopashov, 1951, 1960). This shows the similarity of early processes in both of the eye layers. However, upon their
further divergent development, cytoplasmic components belonging to
the other type of differentiation are eliminated from the internal layer.
In all the animals investigated (Dragomirov, 1935, on newts;
Lopashov, 1960, on various Amphibians; Dabaghian, 1959, on Acipenserid fishes) the possibility of the formation of pigment epithelium from
the internal layer becomes considerably reduced with age, and ceases at
the stage preceding the differentiation of the retina into layers. On the
other hand, though the capacity of the external layer to transform into
retina disappears before its pigmentation in Birds (Alexander, 1937;
Dorris, 1938; Reinbold, 1958), in Acipenserid fishes (Dabaghian, 1959)
and Mammals (Stroeva, 1960) it is preserved for a short time after the
onset of pigmentation of the external layer. In Anurans the pigment epithelium will transform into retina up to the time of metamorphosis
(Lopashov, 1949, 1959; Sato, 1953), while in Urodeles (Wachs, 1920;
Ikeda, 1935; Stone, 1950a, b; Stone and Steinitz, 1957; Reyer, 1956;
Hasegawa, 1958) the power is preserved throughout their whole life.
The regeneration of the retina in Urodeles, and that of the retina and the
lens in newts and salamanders, is based on such metaplasia (Reyer,
1954b).
The proportions of the pigment epithelium and of the retina are not
primarily determined in the eye rudiments. They depend on the ratio of
the internal tension, produced by the fluid which at this stage fills the
cavity common to the brain and eye vesicles, and the pressure of the
parts surrounding the eye. When space is restricted, most of the eye
rudiment is forced into aggregation. This portion differentiates into
retina, localized at the site of contact with the lens. The surface layer of
the eye rudiment, which borders the mesenchyme, develops into pigment epithelium (Lopashov, 1960).
This mechanism of polarization of the eye rudiment does not provide
for the emergence of a regular cup-shaped form of the eye. It leads to
the formation of an irregular aggregation when eye rudiments are
cultivated without the lens either in a saline solution, or in body cavity
fluids (Dorris, 1938; Lopashov, 1945, 1960; Dabaghian, 1958,1959); and
similarly no complete invagination of the eye rudiment is observed in
whole embryos devoid of lenses, with an incomplete contact with the
ectoderm. It also leads to the formation of folds in the retina, to the
evagination of the bottom of the cup towards the ectoderm, and to
inhibition of the development of the iris, anterior chamber and vitreous
chamber (Lehmann, 1934, 1936; Reyer, 1948, 1950). The close adhesion
of the eye vesicle to the lens material, which takes place in normal development, not only contributes to the origin of the retina here, but