366
G. V. LOPASHOV AND O. G.
STROEVA
operating. At the present time three main periods can be clearly distinguished by their characteristics:
1. Inductive effects which separate the specific material of a given
rudiment (eye rudiment within the neural plate, rudiments of the lens
and cornea within the surface ectoderm) determine the first phase of the
development of the organ. If this phase is eliminated, the organ is not
formed at all. In itself, however, this phase is insufficient for the development of the organ. In its general nature, the induction of the eye rudiment, lens and cornea is comparable to that of the nervous system.
2. In the second phase the parts of the rudiment are formed, and
concomitantly the cell differentiation of these parts is determined. This
phase can be performed in different rudiments by different morphogenetic mechanisms independent of those operating in the first phase.
The following mechanisms are clearly distinguished within the eye:
(a) Determination of the type of cell differentiation in relation to the
spatial arrangement of cells within the rudiment and with respect to
the surrounding tissues (the role of the accumulation and flattening of
cells in the segregation of the optic vesicle into retina and pigment
epithelium during the concomitant contact of the latter with the
mesenchyme; and the segregation of the lens rudiment into the epithelial part and the fibre nucleus, upon contact of the latter with the
retina). The formation of the retina at the site of the contact between
the optic rudiment and the lens ectoderm is often regarded as induction.
Here, however, the contact with the lens determines only the localization of the retinal formation, and not the specificity of this process. The
latter is determined by the cell aggregation and by the accumulation of
its own metabolites. On the contrary, if contact of these cells with their
own metabolites is minimal, e.g. should they become arranged in one
layer, preventing the accumulation of metabolites, cell differentiations
go in a different direction, and pigment epithelium arises. This kind of
retinal 'induction' can occur through contact with any other tissue that
enhances the accumulation of cells in the eye rudiment to a mass, as
with the ear vesicle. An interaction of this type clearly differs from
the phenomena going on in the first phase when the separation of the
rudiment proceeds as a result of specific influences which are associated
with the transmission of substances from one rudiment to another
(Flickinger et al., 1959; Vainio et al., 1960). This type of formative processes finds an analogy in the differentiation of the embryonic and
extraembryonic blastoderm in teleost fishes (Luther, 1937), chicks
(Grobstein and Zwilling, 1953) and mice (Grobstein, 1952, 1955b), as
well as in the primary mesoderm segregation in Amphibian embryos
(Muchmore, 1957).
(b) A mechanism, the nature and action of which are still unknown,
G. V. LOPASHOV AND O. G.
STROEVA
operating. At the present time three main periods can be clearly distinguished by their characteristics:
1. Inductive effects which separate the specific material of a given
rudiment (eye rudiment within the neural plate, rudiments of the lens
and cornea within the surface ectoderm) determine the first phase of the
development of the organ. If this phase is eliminated, the organ is not
formed at all. In itself, however, this phase is insufficient for the development of the organ. In its general nature, the induction of the eye rudiment, lens and cornea is comparable to that of the nervous system.
2. In the second phase the parts of the rudiment are formed, and
concomitantly the cell differentiation of these parts is determined. This
phase can be performed in different rudiments by different morphogenetic mechanisms independent of those operating in the first phase.
The following mechanisms are clearly distinguished within the eye:
(a) Determination of the type of cell differentiation in relation to the
spatial arrangement of cells within the rudiment and with respect to
the surrounding tissues (the role of the accumulation and flattening of
cells in the segregation of the optic vesicle into retina and pigment
epithelium during the concomitant contact of the latter with the
mesenchyme; and the segregation of the lens rudiment into the epithelial part and the fibre nucleus, upon contact of the latter with the
retina). The formation of the retina at the site of the contact between
the optic rudiment and the lens ectoderm is often regarded as induction.
Here, however, the contact with the lens determines only the localization of the retinal formation, and not the specificity of this process. The
latter is determined by the cell aggregation and by the accumulation of
its own metabolites. On the contrary, if contact of these cells with their
own metabolites is minimal, e.g. should they become arranged in one
layer, preventing the accumulation of metabolites, cell differentiations
go in a different direction, and pigment epithelium arises. This kind of
retinal 'induction' can occur through contact with any other tissue that
enhances the accumulation of cells in the eye rudiment to a mass, as
with the ear vesicle. An interaction of this type clearly differs from
the phenomena going on in the first phase when the separation of the
rudiment proceeds as a result of specific influences which are associated
with the transmission of substances from one rudiment to another
(Flickinger et al., 1959; Vainio et al., 1960). This type of formative processes finds an analogy in the differentiation of the embryonic and
extraembryonic blastoderm in teleost fishes (Luther, 1937), chicks
(Grobstein and Zwilling, 1953) and mice (Grobstein, 1952, 1955b), as
well as in the primary mesoderm segregation in Amphibian embryos
(Muchmore, 1957).
(b) A mechanism, the nature and action of which are still unknown,