364
G. V. LOPASHOV AND
O. G.
STROEVA
stages in the anterior chamber of the adult eye (Stroeva, 1960). It seems
that, unlike Amphibians, and Urodeles in particular, a considerable
proportion of the cells of the primary envelope of the mammalian eye
migrates directly from the eye rudiment as a supplement to those cells
that take their origin from the neural crest.
The formation of the choroid coat of the eye from the surrounding
mesenchyme takes place in Birds and Mammals only when there is
normal differentiation of the pigment epithelium. In cases of hereditary
(Mann, 1957; Gayer, 1942) and experimental (Giroud et al., 1954;
Giroud, 1957; Stroeva, 1960) colobomas of the retina and the choroid
coat, in the areas where the external layer has differentiated as retina
the choroid coat is lacking. In Mammals there can be present in such
cases only the chorio-capillary layer of the choroid coat, and Bruch's
membrane (Mann, 1957). When the mesenchyme was completely removed from the rat eye rudiment (Stroeva, 1960), it was shown that,
with the onset of pigmentation, the external eye layer acquired a stickiness contributing to a closer contact between the eye and the mesenchyme. It is likely that the lack of stickiness of areas of the external
layer which have differentiated into retina, does not allow the mesenchyme to become organized there into the choroid coat. This question
requires further experimental investigation.
There are no exact experimental data concerning the origin of the
scleral mesenchyme. Differentiation of cartilages in Birds (Weiss and
Amprino, 1940; Coulombre and Coulombre, 1958a) and of the scleral
collagen in Mammals (Smelser and Ozanics, 1956) begins near the corneal
limbus and spreads later to the whole scleral hemisphere of the eye.
Before the onset of chondrification of the sclera in chicks (till the 7th
day of incubation), the loose mesenchyme surrounding the eye does not
differ from the mesenchyme of other regions of the embryo. When
explanted in vitro, up to the 4th day of incubation, this mesenchyme
continues to grow as the usual fibrocytes, while when cultivated after
this period it forms typical scleral cartilages (Weiss and Amprino, 1940).
The sclera differentiates even when a rudimentary eye is represented
merely by a clump of pigment epithelium (Amprino, 1949). Scleral
differentiation, its thickness and appositional growth, directly depend
on the mechanical stretching which arises in association with the intensive eye growth. The stronger the tension, the more the sclera decreases
in thickness. A very strong tension inhibits the differentiation of
the chondrogenic tissue (Weiss and Amprino, 1940). With decrease of
the intraocular tension on the 4th day of incubation, there develop
microphthalmic eyes, the sclera of which are excessively thick (Weiss
and Amprino, 1940; Amprino and Pansa, 1955). In the presclera of such
eyes a condensation of the prescleral mesenchyme takes place, but with
G. V. LOPASHOV AND
O. G.
STROEVA
stages in the anterior chamber of the adult eye (Stroeva, 1960). It seems
that, unlike Amphibians, and Urodeles in particular, a considerable
proportion of the cells of the primary envelope of the mammalian eye
migrates directly from the eye rudiment as a supplement to those cells
that take their origin from the neural crest.
The formation of the choroid coat of the eye from the surrounding
mesenchyme takes place in Birds and Mammals only when there is
normal differentiation of the pigment epithelium. In cases of hereditary
(Mann, 1957; Gayer, 1942) and experimental (Giroud et al., 1954;
Giroud, 1957; Stroeva, 1960) colobomas of the retina and the choroid
coat, in the areas where the external layer has differentiated as retina
the choroid coat is lacking. In Mammals there can be present in such
cases only the chorio-capillary layer of the choroid coat, and Bruch's
membrane (Mann, 1957). When the mesenchyme was completely removed from the rat eye rudiment (Stroeva, 1960), it was shown that,
with the onset of pigmentation, the external eye layer acquired a stickiness contributing to a closer contact between the eye and the mesenchyme. It is likely that the lack of stickiness of areas of the external
layer which have differentiated into retina, does not allow the mesenchyme to become organized there into the choroid coat. This question
requires further experimental investigation.
There are no exact experimental data concerning the origin of the
scleral mesenchyme. Differentiation of cartilages in Birds (Weiss and
Amprino, 1940; Coulombre and Coulombre, 1958a) and of the scleral
collagen in Mammals (Smelser and Ozanics, 1956) begins near the corneal
limbus and spreads later to the whole scleral hemisphere of the eye.
Before the onset of chondrification of the sclera in chicks (till the 7th
day of incubation), the loose mesenchyme surrounding the eye does not
differ from the mesenchyme of other regions of the embryo. When
explanted in vitro, up to the 4th day of incubation, this mesenchyme
continues to grow as the usual fibrocytes, while when cultivated after
this period it forms typical scleral cartilages (Weiss and Amprino, 1940).
The sclera differentiates even when a rudimentary eye is represented
merely by a clump of pigment epithelium (Amprino, 1949). Scleral
differentiation, its thickness and appositional growth, directly depend
on the mechanical stretching which arises in association with the intensive eye growth. The stronger the tension, the more the sclera decreases
in thickness. A very strong tension inhibits the differentiation of
the chondrogenic tissue (Weiss and Amprino, 1940). With decrease of
the intraocular tension on the 4th day of incubation, there develop
microphthalmic eyes, the sclera of which are excessively thick (Weiss
and Amprino, 1940; Amprino and Pansa, 1955). In the presclera of such
eyes a condensation of the prescleral mesenchyme takes place, but with