VIII.
MORPHOGENESIS
OF THE VERTEBRATE
EYE
335
rudiment formation is therefore concentrated in the anterior area of the
neural plate even in the early neurula, but it is not fixed in this area;
it can be changed and suppressed by contact with other parts of the
mesodermal substrate, while the mesoderm underlying the presumptive
eye area can transmit this factor to more posterior parts of the neural
plate.
Experiments at the neurula stage reveal the dual role played by the
anterior part of the mesoderm (prechordal plate) which underlies the
eye-forming area of the neural plate. If the anterior part of the neural
plate is transplanted or explanted without its substrate usually only
one eye appears in it; but if it is taken along with its substrate, two
eyes arise (Adelmann, 1930, 1936, 1937; Alderman, 1935; Mikami,
1941b; von Woellwarth, 1952; Waechter, 1953). Cyclopia of various
degrees regularly occurs upon the removal of the whole anterio-cranial
mesoderm, and particularly of its median part (prechordal plate)
(Mangold, 1931, 1936, 1957; Adelmann, 1936, 1937). If the substrate is
rotated the axis of the head is correspondingly shifted (Alderman, 1935,
1938; Adelmann, 1937; Mikami, 1941b). It seems that the median
portion of the mesoderm at first contributes most actively to the
development of the eye material, and then inhibits its formation in the
middle of the brain floor. This is why two eyes appear.
Similar changes leading to cyclopia take place not only as a result of
the operative removal of the prechordal mesoderm but also as a result
of conditions suppressing, in the gastrulation period, the development
of the prechordal plate and the completeness of invagination: in particular, the action of LiCl (Leplat, 1920; Adelmann, 1934, 1936;
Lehmann, 1938, 1945; Tamini, 1943; Gallien and Signoret, 1957) and
of oxygen deficiency in Amphibians (Rübsaamen, 1949, 1955; Mangold
and Waechter, 1953, Tiedemann and Tiedemann, 1954; Mangold, 1957).
The action of NaSCN, which leads to an overdevelopment of the prechordal plate (Ranzi and Tamini, 1939; Ranzi, 1945), produces a
correspondingly excessive development of the material of the eye
rudiments; due to this, the latter do not separate, and join under the
brain, or the retina in the enlarged eyes forms folds and accessory
invaginations (Corti, 1950).
These data give only a first approach towards an understanding of the
mechanism of the primary origin of the eye rudiments. In order to
understand this better, it should be considered in relation to general
processes of the development of parts of the brain, and, in particular, to
the sequence of the processes occurring there in several phases.
Transplantations (Gallera, 1947, 1948; Dragomirova, 1949; EyalGiladi, 1954) and explantations (Mikami, 1941b; Gallera, 1948; von
Woellwarth, 1952) of the presumptive eye area of the gastrula ectoderm
MORPHOGENESIS
OF THE VERTEBRATE
EYE
335
rudiment formation is therefore concentrated in the anterior area of the
neural plate even in the early neurula, but it is not fixed in this area;
it can be changed and suppressed by contact with other parts of the
mesodermal substrate, while the mesoderm underlying the presumptive
eye area can transmit this factor to more posterior parts of the neural
plate.
Experiments at the neurula stage reveal the dual role played by the
anterior part of the mesoderm (prechordal plate) which underlies the
eye-forming area of the neural plate. If the anterior part of the neural
plate is transplanted or explanted without its substrate usually only
one eye appears in it; but if it is taken along with its substrate, two
eyes arise (Adelmann, 1930, 1936, 1937; Alderman, 1935; Mikami,
1941b; von Woellwarth, 1952; Waechter, 1953). Cyclopia of various
degrees regularly occurs upon the removal of the whole anterio-cranial
mesoderm, and particularly of its median part (prechordal plate)
(Mangold, 1931, 1936, 1957; Adelmann, 1936, 1937). If the substrate is
rotated the axis of the head is correspondingly shifted (Alderman, 1935,
1938; Adelmann, 1937; Mikami, 1941b). It seems that the median
portion of the mesoderm at first contributes most actively to the
development of the eye material, and then inhibits its formation in the
middle of the brain floor. This is why two eyes appear.
Similar changes leading to cyclopia take place not only as a result of
the operative removal of the prechordal mesoderm but also as a result
of conditions suppressing, in the gastrulation period, the development
of the prechordal plate and the completeness of invagination: in particular, the action of LiCl (Leplat, 1920; Adelmann, 1934, 1936;
Lehmann, 1938, 1945; Tamini, 1943; Gallien and Signoret, 1957) and
of oxygen deficiency in Amphibians (Rübsaamen, 1949, 1955; Mangold
and Waechter, 1953, Tiedemann and Tiedemann, 1954; Mangold, 1957).
The action of NaSCN, which leads to an overdevelopment of the prechordal plate (Ranzi and Tamini, 1939; Ranzi, 1945), produces a
correspondingly excessive development of the material of the eye
rudiments; due to this, the latter do not separate, and join under the
brain, or the retina in the enlarged eyes forms folds and accessory
invaginations (Corti, 1950).
These data give only a first approach towards an understanding of the
mechanism of the primary origin of the eye rudiments. In order to
understand this better, it should be considered in relation to general
processes of the development of parts of the brain, and, in particular, to
the sequence of the processes occurring there in several phases.
Transplantations (Gallera, 1947, 1948; Dragomirova, 1949; EyalGiladi, 1954) and explantations (Mikami, 1941b; Gallera, 1948; von
Woellwarth, 1952) of the presumptive eye area of the gastrula ectoderm