VIII.
MORPHOGENESIS
OF THE
VERTEBRATE
EYE
357
experimental data on corneal induction in amphibians and birds together, though it is unlikely that the processes are absolutely identical.
One may assume that the transparent protective 'spectacles' of snakes
and high-speed swimming fishes also arise under the influence of the
eye; the character of the interaction between the eye and the lids,
however, would certainly differ from that between the eye and the
ectoderm which gives rise to the cornea.
Since a detailed analysis of early papers dealing with corneal induction is presented by Mangold (1931), we shall dwell on them but briefly.
After removal of the presumptive eye at the neurula stage (Spemann,
1901), or of the optic vesicle at the tail-bud stage (Lewis, 1905), the
presumptive corneal ectoderm on the operated side transforms into the
usual surface epidermis. Amprino (1949) showed the same in chicks.
After transplantation of epidermis from other regions of the larval body
to the position of the excised cornea (Groll, 1923), or after transplantation of the eye rudiment to an unusual site (Fischel, 1917, 1919; Adelmann, 1928), the epidermis above the eye becomes depigmented and
transforms to transparent cornea. The same happens to epidermis
migrating over the wound after simple removal of the external cornea
(Lewis, 1905; Dürken, 1913; Popoff, 1938). Upon implantation of the
corneal ectoderm of Ambystoma punctatum (covering the eye vesicle,
the early lens vesicle and the eye cup) in the eye chamber after removal
of the lens, incompletely differentiated corneal vesicles were formed;
while after transplantation into the caudal fin, epidermis with Leydig's
cells was formed (Reyer, 1957). Upon transplantation of an eye cup with
lens, without the presumptive corneal epithelium, under the flank
ectoderm of another embryo (donor age 2^-3 days; host age 2-2| days),
the ectoderm above the eye was transformed into cornea, if the pupil
of the eye faced it (Neyfakh, 1950). Corneal induction by the eye was
also successful in culture on the chorioallantois after the cornea of the
eye had been replaced by skin epithelium (host age 3-4 and 4-5 days).
The developmental age of the cornea thus formed depended on the
amount of time the skin epithelium had been in contact with the eye
but not on the actual age of either epidermis or eye (Neyfakh, 1952b).
In a number of cases where the lens and the retina lay side by side, in
parallel to the epithelium, the cornea arose only over the lens. Special
experiments revealed, however, that neither the isolated lens alone, nor
the eye cup alone without the lens, can evoke corneal characters in the
skin (Neyfakh, 1952b). These experiments have not confirmed the suggestion of Amprino (1949) that the cornea can be induced by the lens
alone. The cornea arises only when the lens is connected with an area of
the retina however small (Neyfakh, 1952b). In contrast, the external
cornea of Amphibians can be induced by either the lens alone or the eye
MORPHOGENESIS
OF THE
VERTEBRATE
EYE
357
experimental data on corneal induction in amphibians and birds together, though it is unlikely that the processes are absolutely identical.
One may assume that the transparent protective 'spectacles' of snakes
and high-speed swimming fishes also arise under the influence of the
eye; the character of the interaction between the eye and the lids,
however, would certainly differ from that between the eye and the
ectoderm which gives rise to the cornea.
Since a detailed analysis of early papers dealing with corneal induction is presented by Mangold (1931), we shall dwell on them but briefly.
After removal of the presumptive eye at the neurula stage (Spemann,
1901), or of the optic vesicle at the tail-bud stage (Lewis, 1905), the
presumptive corneal ectoderm on the operated side transforms into the
usual surface epidermis. Amprino (1949) showed the same in chicks.
After transplantation of epidermis from other regions of the larval body
to the position of the excised cornea (Groll, 1923), or after transplantation of the eye rudiment to an unusual site (Fischel, 1917, 1919; Adelmann, 1928), the epidermis above the eye becomes depigmented and
transforms to transparent cornea. The same happens to epidermis
migrating over the wound after simple removal of the external cornea
(Lewis, 1905; Dürken, 1913; Popoff, 1938). Upon implantation of the
corneal ectoderm of Ambystoma punctatum (covering the eye vesicle,
the early lens vesicle and the eye cup) in the eye chamber after removal
of the lens, incompletely differentiated corneal vesicles were formed;
while after transplantation into the caudal fin, epidermis with Leydig's
cells was formed (Reyer, 1957). Upon transplantation of an eye cup with
lens, without the presumptive corneal epithelium, under the flank
ectoderm of another embryo (donor age 2^-3 days; host age 2-2| days),
the ectoderm above the eye was transformed into cornea, if the pupil
of the eye faced it (Neyfakh, 1950). Corneal induction by the eye was
also successful in culture on the chorioallantois after the cornea of the
eye had been replaced by skin epithelium (host age 3-4 and 4-5 days).
The developmental age of the cornea thus formed depended on the
amount of time the skin epithelium had been in contact with the eye
but not on the actual age of either epidermis or eye (Neyfakh, 1952b).
In a number of cases where the lens and the retina lay side by side, in
parallel to the epithelium, the cornea arose only over the lens. Special
experiments revealed, however, that neither the isolated lens alone, nor
the eye cup alone without the lens, can evoke corneal characters in the
skin (Neyfakh, 1952b). These experiments have not confirmed the suggestion of Amprino (1949) that the cornea can be induced by the lens
alone. The cornea arises only when the lens is connected with an area of
the retina however small (Neyfakh, 1952b). In contrast, the external
cornea of Amphibians can be induced by either the lens alone or the eye