VIII.
MORPHOGENESIS
OF THE VERTEBRATE
EYE
337
appearance of its other parts. However, although in normal development, the neural rudiment arises as a result of the passage of such an
agent from the mesoderm into the ectoderm, after injuring the latter a
release of the neurogenic factor may start within its cells (Holtfreter,
1944, 1945, 1948; Karasaki, 1957; Okano and Kawakami, 1959). This
makes these cells develop only forebrain structures with eyes. In this
case the agents seem to act in the same cells where they are formed, as
mesodermal substances do during the development of the mesoderm
(though mesodermal cells can also arise after the transfer of mesodermal
factors from other cells—ter Horst, 1948; Spofford, 1948; and from
alien inductors—Toivonen, 1953, 1954, 1958; Yamada, 1958a, b).
The data obtained by Takaya (1955) and by Johnen (1956a, b) can
most probably be interpreted by the suggestion that the neurogenic
agent passes to the ectoderm more rapidly than the mesodermal. The
first phase is realized during gastrulation, when ectodermal cells,
evidently in relation to the prevalence of either neurogenic or mesodermal factors, form neural or mesodermal cells respectively, and in a
greater or lesser amount. This phase is limited by ageing of the cells
(Holtfreter, 1938; Gallera, 1952).
During the second phase the eye-forming properties of the already
emerged neural plate increase under the prolonged action of the
mesodermal substrate; at the same time the inhibiting influence of the
prechordal plate, which leads to the division of the eye rudiment into
two areas, begins to manifest itself in the median area of the neural
plate. A sharp opposition between the two phases (Nieuwkoop, 1955,
1958) which makes absolute the autonomous tendencies in the development of the ectoderm and neural plate can hardly correctly mirror
(cf. Toivonen, 1958; Lopashov, 1960) the peculiarities of either of them.
The forebrain region with eye rudiments acquires at each stage
certain developmental capacities which are realized independently of the
mesodermal substrate with which the appearance of these capacities is
connected. Upon injury to the gastrula ectoderm, there arise in it parts
of the forebrain, which possess spherical symmetry, with eyes arranged
on the surface of the roundish brain (Holtfreter, 1944). After experimental separation of the eye area of the ectoderm from the axial mesoderm at the gastrula stage, a neural rudiment is formed from it, with
one eye in front; while after a longer (to the end of neurulation) or a
continuous connection with the substrate, there arises a bilateral brain
with two eyes. The size of the eye rudiment arising then turns out to be
dependent mainly upon the size of the mesodermal substrate which
comes into contact with the ectoderm, and not upon the species-specific
properties of the ectoderm. This is demonstrated by experiments involving reciprocal transplantation of the ectodermal areas of the gastrula
MORPHOGENESIS
OF THE VERTEBRATE
EYE
337
appearance of its other parts. However, although in normal development, the neural rudiment arises as a result of the passage of such an
agent from the mesoderm into the ectoderm, after injuring the latter a
release of the neurogenic factor may start within its cells (Holtfreter,
1944, 1945, 1948; Karasaki, 1957; Okano and Kawakami, 1959). This
makes these cells develop only forebrain structures with eyes. In this
case the agents seem to act in the same cells where they are formed, as
mesodermal substances do during the development of the mesoderm
(though mesodermal cells can also arise after the transfer of mesodermal
factors from other cells—ter Horst, 1948; Spofford, 1948; and from
alien inductors—Toivonen, 1953, 1954, 1958; Yamada, 1958a, b).
The data obtained by Takaya (1955) and by Johnen (1956a, b) can
most probably be interpreted by the suggestion that the neurogenic
agent passes to the ectoderm more rapidly than the mesodermal. The
first phase is realized during gastrulation, when ectodermal cells,
evidently in relation to the prevalence of either neurogenic or mesodermal factors, form neural or mesodermal cells respectively, and in a
greater or lesser amount. This phase is limited by ageing of the cells
(Holtfreter, 1938; Gallera, 1952).
During the second phase the eye-forming properties of the already
emerged neural plate increase under the prolonged action of the
mesodermal substrate; at the same time the inhibiting influence of the
prechordal plate, which leads to the division of the eye rudiment into
two areas, begins to manifest itself in the median area of the neural
plate. A sharp opposition between the two phases (Nieuwkoop, 1955,
1958) which makes absolute the autonomous tendencies in the development of the ectoderm and neural plate can hardly correctly mirror
(cf. Toivonen, 1958; Lopashov, 1960) the peculiarities of either of them.
The forebrain region with eye rudiments acquires at each stage
certain developmental capacities which are realized independently of the
mesodermal substrate with which the appearance of these capacities is
connected. Upon injury to the gastrula ectoderm, there arise in it parts
of the forebrain, which possess spherical symmetry, with eyes arranged
on the surface of the roundish brain (Holtfreter, 1944). After experimental separation of the eye area of the ectoderm from the axial mesoderm at the gastrula stage, a neural rudiment is formed from it, with
one eye in front; while after a longer (to the end of neurulation) or a
continuous connection with the substrate, there arises a bilateral brain
with two eyes. The size of the eye rudiment arising then turns out to be
dependent mainly upon the size of the mesodermal substrate which
comes into contact with the ectoderm, and not upon the species-specific
properties of the ectoderm. This is demonstrated by experiments involving reciprocal transplantation of the ectodermal areas of the gastrula