84
ALFRED J. COULOMBRE
eye cup immediately after the lens vesicle has separated from the surface ectoderm. It becomes two-layered by the addition of a superficial
layer of squamous cells to the original cuboidal ectoderm layer either at
the time of closure of the lens pore (chick: O'Rahilly and Meyer, 1959)
or shortly thereafter (Sauropsida: Mann, 1950). Experiments with
Amphibia (Spemann, 1901; Lewis, 1904, 1905) and chick embryos
(Amprino, 1949; Neifach, 1952b) have demonstrated that the influence
of the lens and optic cup is necessary for thç formation of the cornea.
The concept that the lens and optic cup induce the cornea is further
supported by the ability of these structures to elicit cornea from other
ectodermal regions (Amphibia: Lewis, 1905; Dürken, 1913; Fischel,
1917, 1919; Groll, 1923; Adelmann, 1928; Popov, 1938; Reyer, 1950;
Ignatieva, 1951, 1952; chick embryo: Neifach, 1950, 1952b). These
experiments also demonstrated that the capacity to form cornea is
widespread in the early ectoderm. Yet the cornea normally develops
only over the eye and in precise alignment with the other dioptric
media. This is explained by the observation that it is induced only by
the anterior portion of the intact eye (Neifach, 1950). It is of an appropriate size because its area is dictated by the region of contact between
the lens (or the anterior opening of the optic cup) and the presumptive
corneal ectoderm (Lewis, 1905; Fischel, 1917; Lazarev, 1946; Reyer,
1950). These same investigations revealed that in the Amphibia either
lens or eye cup alone can effect corneal induction. In the chick embryo
Amprino (1949) reported that lens alone could induce cornea. On the
basis of the differentiation of chorioallantoic grafts of embryonic eye
wall Neifach (1952b) suggested that the lens did not induce cornea
unless at least a small piece of retina was present. Even in Amphibia
where the eye cup alone can initiate corneal differentiation, the cornea
remains in a primitive state unless and until a lens is present to support
its full development (Reyer, 1950). Thus, while the eye cup alone can
induce cornea in Amphibia, and while it appears to be necessary, but
not sufficient, to induce cornea in the chick embryo, the principal influence determining the size, shape, and position of the cornea seems to
emanate from the lens. In most forms the eye, as it becomes older, loses
its ability to induce cornea in competent embryonic ectoderm (Bednyakova, 1954,1955; Ignatieva, 1955a, b; Beliaeva, 1955).
The cornea is not only induced by the lens and eye cup but it also
remains dependent upon them for some time for its maintenance (fish :
Harms, 1914; Amphibia: Lewis, 1905; Dürken, 1913; Fischel, 1917;
Groll, 1923; Beliaeva, 1950; birds: Neifach, 1952c; Bytinski-Salz, 1960,
1961; mammals: Stroeva, 1956). Anuran tadpoles and some fishes
possess an external cornea which is continuous with the skin, and an
internal cornea which is continuous with the sciera. In anurans these
ALFRED J. COULOMBRE
eye cup immediately after the lens vesicle has separated from the surface ectoderm. It becomes two-layered by the addition of a superficial
layer of squamous cells to the original cuboidal ectoderm layer either at
the time of closure of the lens pore (chick: O'Rahilly and Meyer, 1959)
or shortly thereafter (Sauropsida: Mann, 1950). Experiments with
Amphibia (Spemann, 1901; Lewis, 1904, 1905) and chick embryos
(Amprino, 1949; Neifach, 1952b) have demonstrated that the influence
of the lens and optic cup is necessary for thç formation of the cornea.
The concept that the lens and optic cup induce the cornea is further
supported by the ability of these structures to elicit cornea from other
ectodermal regions (Amphibia: Lewis, 1905; Dürken, 1913; Fischel,
1917, 1919; Groll, 1923; Adelmann, 1928; Popov, 1938; Reyer, 1950;
Ignatieva, 1951, 1952; chick embryo: Neifach, 1950, 1952b). These
experiments also demonstrated that the capacity to form cornea is
widespread in the early ectoderm. Yet the cornea normally develops
only over the eye and in precise alignment with the other dioptric
media. This is explained by the observation that it is induced only by
the anterior portion of the intact eye (Neifach, 1950). It is of an appropriate size because its area is dictated by the region of contact between
the lens (or the anterior opening of the optic cup) and the presumptive
corneal ectoderm (Lewis, 1905; Fischel, 1917; Lazarev, 1946; Reyer,
1950). These same investigations revealed that in the Amphibia either
lens or eye cup alone can effect corneal induction. In the chick embryo
Amprino (1949) reported that lens alone could induce cornea. On the
basis of the differentiation of chorioallantoic grafts of embryonic eye
wall Neifach (1952b) suggested that the lens did not induce cornea
unless at least a small piece of retina was present. Even in Amphibia
where the eye cup alone can initiate corneal differentiation, the cornea
remains in a primitive state unless and until a lens is present to support
its full development (Reyer, 1950). Thus, while the eye cup alone can
induce cornea in Amphibia, and while it appears to be necessary, but
not sufficient, to induce cornea in the chick embryo, the principal influence determining the size, shape, and position of the cornea seems to
emanate from the lens. In most forms the eye, as it becomes older, loses
its ability to induce cornea in competent embryonic ectoderm (Bednyakova, 1954,1955; Ignatieva, 1955a, b; Beliaeva, 1955).
The cornea is not only induced by the lens and eye cup but it also
remains dependent upon them for some time for its maintenance (fish :
Harms, 1914; Amphibia: Lewis, 1905; Dürken, 1913; Fischel, 1917;
Groll, 1923; Beliaeva, 1950; birds: Neifach, 1952c; Bytinski-Salz, 1960,
1961; mammals: Stroeva, 1956). Anuran tadpoles and some fishes
possess an external cornea which is continuous with the skin, and an
internal cornea which is continuous with the sciera. In anurans these