356
G. V. LOPASHOV AND
O. G. STROEVA
external injury. The definitive cornea in most terrestrial Vertebrates
and in man consists of five layers: anterior corneal epithelium, external
limiting (Bowman's) membrane, substantia propria, internal limiting
(Descement's) membrane, and the internal Descement's epithelium.
The cornea is of convex shape and is delimited from the lens and the iris
by the anterior chamber.
The cornea is formed after the lens detaches from the epidermis,
which up to this time has shown no differences from the remaining surface epithelium. From the anatomical aspect, corneal formation proceeds in a different way in animals of different systematic and ecological
groups. Anuran tadpoles and some fishes possess two corneas: an internal
one which is a continuation of the larval sclera, and an external one that is
a modification of the larval skin. The corneas are separated by a cavity,
so that the eye-ball turns freely under the immobile external cornea.
The latter represents a transparent window upon the eye and is directly
transitional to the surrounding skin, differing from it only in a lesser
thickness, and in lack of pigment cells and skin glands, Ebert's structures
(characteristic of the epidermis of Anuran tadpoles) or Leydig's cells
(characteristic of the epidermis of Urodelan larvae). The definitive
cornea of Anurans is formed at the time of metamorphosis through the
fusion of the external and internal larval corneas (Harms, 1923; Popoff,
1938). In some fishes (demersal) two corneas are preserved throughout
life. In many Reptiles, in snakes in particular, there is formed yet
another pair of 'spectacles', a transparent window in the lower lid after
a complete fusion of the lids over the eye; while in high-speed swimming
fishes there is a similar structure formed from the vertical lid (Walls,
1942). In human beings (Mann, 1949), dogs (Medvedeva, 1955), as well
as in chicks (Ladyjenskaia, 1917; Laguesse, 1926; O'Rahilly and Meyer,
1959; Meyer and O'Rahilly, 1959) the cornea arises primarily in the form
of a rudiment which consists of the anterior epithelium and the cell-free
postepithelial layer; the latter, having been invaded by mesenchyme
cells, differentiates into the substantia propria of the definitive cornea.
A detailed description of the histogenesis of the chick cornea, including
a comparison with data obtained by previous workers and with development in man is presented by O'Rahilly and Meyer (1959) and Meyer
and O'Rahilly (1959). In rats (Medvedeva, 1955) and in rabbits (Rabl,
1900) the anterior epithelium of the cornea is from the beginning
underlain by mesenchyme cells.
Though there exist various types of cornea formation, it seems possible to assume for all of them that the cornea arises in the surface
ectoderm under the influence of the eye cup (corneal induction).
Spemann (1901) and Lewis (1904, 1905) were the first to show this in
amphibian larvae, and Neyfakh (1950) in the chick. We will consider
G. V. LOPASHOV AND
O. G. STROEVA
external injury. The definitive cornea in most terrestrial Vertebrates
and in man consists of five layers: anterior corneal epithelium, external
limiting (Bowman's) membrane, substantia propria, internal limiting
(Descement's) membrane, and the internal Descement's epithelium.
The cornea is of convex shape and is delimited from the lens and the iris
by the anterior chamber.
The cornea is formed after the lens detaches from the epidermis,
which up to this time has shown no differences from the remaining surface epithelium. From the anatomical aspect, corneal formation proceeds in a different way in animals of different systematic and ecological
groups. Anuran tadpoles and some fishes possess two corneas: an internal
one which is a continuation of the larval sclera, and an external one that is
a modification of the larval skin. The corneas are separated by a cavity,
so that the eye-ball turns freely under the immobile external cornea.
The latter represents a transparent window upon the eye and is directly
transitional to the surrounding skin, differing from it only in a lesser
thickness, and in lack of pigment cells and skin glands, Ebert's structures
(characteristic of the epidermis of Anuran tadpoles) or Leydig's cells
(characteristic of the epidermis of Urodelan larvae). The definitive
cornea of Anurans is formed at the time of metamorphosis through the
fusion of the external and internal larval corneas (Harms, 1923; Popoff,
1938). In some fishes (demersal) two corneas are preserved throughout
life. In many Reptiles, in snakes in particular, there is formed yet
another pair of 'spectacles', a transparent window in the lower lid after
a complete fusion of the lids over the eye; while in high-speed swimming
fishes there is a similar structure formed from the vertical lid (Walls,
1942). In human beings (Mann, 1949), dogs (Medvedeva, 1955), as well
as in chicks (Ladyjenskaia, 1917; Laguesse, 1926; O'Rahilly and Meyer,
1959; Meyer and O'Rahilly, 1959) the cornea arises primarily in the form
of a rudiment which consists of the anterior epithelium and the cell-free
postepithelial layer; the latter, having been invaded by mesenchyme
cells, differentiates into the substantia propria of the definitive cornea.
A detailed description of the histogenesis of the chick cornea, including
a comparison with data obtained by previous workers and with development in man is presented by O'Rahilly and Meyer (1959) and Meyer
and O'Rahilly (1959). In rats (Medvedeva, 1955) and in rabbits (Rabl,
1900) the anterior epithelium of the cornea is from the beginning
underlain by mesenchyme cells.
Though there exist various types of cornea formation, it seems possible to assume for all of them that the cornea arises in the surface
ectoderm under the influence of the eye cup (corneal induction).
Spemann (1901) and Lewis (1904, 1905) were the first to show this in
amphibian larvae, and Neyfakh (1950) in the chick. We will consider