VIII.
MORPHOGENESIS
OF THE VERTEBRATE EYE
369
coat, are too scarce to allow one to speak of the type of their development.
Such subdivision into phases is undoubtedly schematic to some degree.
Actually all these numerous processes are realized on the background
of a much greater number of relations such as temporal changes of the
properties of tissues participating in development which strongly differ
in different species. In the present state of the problem, however, this
scheme seems to us to be fruitful. It allows us not only to demonstrate
the incompleteness of our knowledge of the development of this or that
rudiment, but it also shows what type of dependence should be looked
for and what is to be expected at this or that phase of a rudiment's
development. The mechanisms analysed are not peculiar to the eye
only. Each of them finds analogies in the development of other organs,
and this enables us to compare various organogeneses and, in the long
run, to better understand general morphogenetical mechanisms in
embryogenesis.
To the phenomena discussed above one should add a number of other
problems, not yet solved, but requiring new approaches for their solution.
The investigations of eye development clearly show that the mechanisms which determine the development, though possessing some features
in common, are at the same time not the same in different species,
especially, in larger systematic groups. These differences are associated
with the shifts of individual phenomena from phase to phase, and with
ecological peculiarities of the species. This is well stated in the descriptive methods but is insufficiently considered in the analysis of morphogenetical interrelations. The available experimental studies are so far
unable to explain the development of adaptive structural differences
found in different systematic groups.
Are the processes of eye morphogenesis connected with the action of
light, or, in other words, what role does specific function play in the
development of this organ? A whole number of works show that in
animals with well developed vision the development of the eye proceeds
normally in the dark (Ognev, 1910; Baburina, 1948; Oepen, 1950).
Moreover, in some Birds and Mammals development proceeds not only
in the dark but, already in the uterus, the lids of foetuses fuse and in
some forms their eyes develop under the protection of closed lids for
some time after their birth. This shows that with the complication of
eye organization the period of eye formation and of its protection against
a too early onset of function is lengthened. But in cave Fishes and
Amphibians with reduced eyes the state is different: at the beginning of
development their eyes are relatively normal, but degenerate during
development. In some forms such as Anoptichthys (Ltiling, 1955) this
happens during development in light, while in Proteus and Typhlotriton
(Kammerer, 1912; Noble and Pope, 1928; Vandel and Bouillon, 1959)
MORPHOGENESIS
OF THE VERTEBRATE EYE
369
coat, are too scarce to allow one to speak of the type of their development.
Such subdivision into phases is undoubtedly schematic to some degree.
Actually all these numerous processes are realized on the background
of a much greater number of relations such as temporal changes of the
properties of tissues participating in development which strongly differ
in different species. In the present state of the problem, however, this
scheme seems to us to be fruitful. It allows us not only to demonstrate
the incompleteness of our knowledge of the development of this or that
rudiment, but it also shows what type of dependence should be looked
for and what is to be expected at this or that phase of a rudiment's
development. The mechanisms analysed are not peculiar to the eye
only. Each of them finds analogies in the development of other organs,
and this enables us to compare various organogeneses and, in the long
run, to better understand general morphogenetical mechanisms in
embryogenesis.
To the phenomena discussed above one should add a number of other
problems, not yet solved, but requiring new approaches for their solution.
The investigations of eye development clearly show that the mechanisms which determine the development, though possessing some features
in common, are at the same time not the same in different species,
especially, in larger systematic groups. These differences are associated
with the shifts of individual phenomena from phase to phase, and with
ecological peculiarities of the species. This is well stated in the descriptive methods but is insufficiently considered in the analysis of morphogenetical interrelations. The available experimental studies are so far
unable to explain the development of adaptive structural differences
found in different systematic groups.
Are the processes of eye morphogenesis connected with the action of
light, or, in other words, what role does specific function play in the
development of this organ? A whole number of works show that in
animals with well developed vision the development of the eye proceeds
normally in the dark (Ognev, 1910; Baburina, 1948; Oepen, 1950).
Moreover, in some Birds and Mammals development proceeds not only
in the dark but, already in the uterus, the lids of foetuses fuse and in
some forms their eyes develop under the protection of closed lids for
some time after their birth. This shows that with the complication of
eye organization the period of eye formation and of its protection against
a too early onset of function is lengthened. But in cave Fishes and
Amphibians with reduced eyes the state is different: at the beginning of
development their eyes are relatively normal, but degenerate during
development. In some forms such as Anoptichthys (Ltiling, 1955) this
happens during development in light, while in Proteus and Typhlotriton
(Kammerer, 1912; Noble and Pope, 1928; Vandel and Bouillon, 1959)