VIII.
MORPHOGENESIS
OF THE VERTEBRATE
EYE
353
that in newts lenses can regenerate from the iris throughout life,
Spemann (1905) and Mangold (1931) suggested that in the adult eye
there occurs a process analogous to the lens induction during embryonic
development and that the retina, possibly, preserves its inductive
ability throughout ontogenesis. This suggestion was confirmed in the
case of lens production from the iris in newts (Stone, 1958a, b; Stone
and Gallacher, 1958). It remained unclear, however: (1) whether adult
eyes would act on the embryonic ectoderm as well; (2) whether the
situation holds not only for newts but for other Vertebrates. Various
authors described transformation into lenses of the following tissues,
after transplanting them into the cavity of the eye cup of embryos and
larvae: epidermis of various ages (Popoff, 1937, 1938, 1939a, b; Popoff
et al., 1939; Nikitenko, 1937; Manuilova, 1938, 1939, 1940); eye
and brain anläge (Popoff, 1937); epidermis of resorbing tadpole tail
(Sikharulidze, 1954); nasal placode (Ikeda, 1938); cornea at early stages
(Ikeda, 1939; Neyfakh, 1948); regeneration blastemas (Schotte and
Hummel, 1939); pigment epithelium of tadpoles (Sato, 1953). Later
Popoff (1940, 1945, 1948) reported such transformation also when epidermis was transplanted into the eye of adult Anurans. These experiments, however, were subjected to severe criticism (Stone and Sapir,
1938, 1940; Okada, 1939, 1943a, b; Reyer, 1954b, 1956, 1958a; Zalokar,
1955; Vereiskaia, 1954a, b, 1957). If lenses or their rudiments are not
sufficiently completely removed, they soon produce new lenses (Stone
and Sapir, 1940; Okada, 1939, 1943a, b; Ikeda, 1934; Reyer, 1950,
1954b) which can be taken for induced ones. Control experiments on
lens 'removal' in tadpoles in which it regenerated (Manuilova et al.,
1938) clearly show the questionable validity of conclusions for this stage
as well. The fusion of the remainder of the epidermal transplant with
the lens cannot serve as a criterion of lens origin, since they actively
merge together (Stone and Sapir, 1938; Vereiskaia, 1957). When the
experiments were repeated, they did not confirm the possibility of lens
formation from blastemas (Stone and Sapir, 1938; Emerson, 1940), or
of transformation into lens of embryo skin when implanted into the
eye of adult frogs and rats (Vereiskaia, 1954a, b, 1957). The latter
conclusion finds support in those experiments where the eye of adult
Anurans and Mammals is used as a chamber for the cultivation of a
number of rudiments and provides for their typical differentiation.
Thus, only the experiments with early developmental stages of the
eye cup remain fairly reliable. The retina seems to be capable of inducing
lenses from embryonic epidermis, presumptive corneal ectoderm, and
nasal placodes when implanted into the eye cavity only prior to the
stage of retinal segregation into layers. Experiments on transplantation
under the ectoderm of previously explanted eye rudiments in Amphi-
MORPHOGENESIS
OF THE VERTEBRATE
EYE
353
that in newts lenses can regenerate from the iris throughout life,
Spemann (1905) and Mangold (1931) suggested that in the adult eye
there occurs a process analogous to the lens induction during embryonic
development and that the retina, possibly, preserves its inductive
ability throughout ontogenesis. This suggestion was confirmed in the
case of lens production from the iris in newts (Stone, 1958a, b; Stone
and Gallacher, 1958). It remained unclear, however: (1) whether adult
eyes would act on the embryonic ectoderm as well; (2) whether the
situation holds not only for newts but for other Vertebrates. Various
authors described transformation into lenses of the following tissues,
after transplanting them into the cavity of the eye cup of embryos and
larvae: epidermis of various ages (Popoff, 1937, 1938, 1939a, b; Popoff
et al., 1939; Nikitenko, 1937; Manuilova, 1938, 1939, 1940); eye
and brain anläge (Popoff, 1937); epidermis of resorbing tadpole tail
(Sikharulidze, 1954); nasal placode (Ikeda, 1938); cornea at early stages
(Ikeda, 1939; Neyfakh, 1948); regeneration blastemas (Schotte and
Hummel, 1939); pigment epithelium of tadpoles (Sato, 1953). Later
Popoff (1940, 1945, 1948) reported such transformation also when epidermis was transplanted into the eye of adult Anurans. These experiments, however, were subjected to severe criticism (Stone and Sapir,
1938, 1940; Okada, 1939, 1943a, b; Reyer, 1954b, 1956, 1958a; Zalokar,
1955; Vereiskaia, 1954a, b, 1957). If lenses or their rudiments are not
sufficiently completely removed, they soon produce new lenses (Stone
and Sapir, 1940; Okada, 1939, 1943a, b; Ikeda, 1934; Reyer, 1950,
1954b) which can be taken for induced ones. Control experiments on
lens 'removal' in tadpoles in which it regenerated (Manuilova et al.,
1938) clearly show the questionable validity of conclusions for this stage
as well. The fusion of the remainder of the epidermal transplant with
the lens cannot serve as a criterion of lens origin, since they actively
merge together (Stone and Sapir, 1938; Vereiskaia, 1957). When the
experiments were repeated, they did not confirm the possibility of lens
formation from blastemas (Stone and Sapir, 1938; Emerson, 1940), or
of transformation into lens of embryo skin when implanted into the
eye of adult frogs and rats (Vereiskaia, 1954a, b, 1957). The latter
conclusion finds support in those experiments where the eye of adult
Anurans and Mammals is used as a chamber for the cultivation of a
number of rudiments and provides for their typical differentiation.
Thus, only the experiments with early developmental stages of the
eye cup remain fairly reliable. The retina seems to be capable of inducing
lenses from embryonic epidermis, presumptive corneal ectoderm, and
nasal placodes when implanted into the eye cavity only prior to the
stage of retinal segregation into layers. Experiments on transplantation
under the ectoderm of previously explanted eye rudiments in Amphi-