338
G.
V. LOPASHOV AND O. G.
STROEVA
from which the forebrain and eye rudiment are later formed, between
anuran species of which the embryos and their eyes differ considerably
in size (Balinsky, 1958).
The marking out of eye rudiments within the neural plate leads later
to their separation from the brain; these take part in this separation the
phenomena of negative affinity which arises between the material of the
eye rudiment and that of the remaining brain (Holtfreter, 1939). These
processes, however, do not provide for the further normal formation of
the eye rudiments; other conditions are required for the achievement of
the typical structure of the eye. These conditions act during the following
stage of development, and will be discussed in the next section.
III. Mechanisms of Segregation and Organization
of the Primary Eye Rudiment
By the end of neurulation the eye rudiments transform into vesicles,
paired outgrowths of the forebrain. In most Vertebrates they are hollow,
their cavity being connected with that of the brain through the eye
stalk. Increasing in size, eye vesicles come into contact with the ectoderm, in which they induce the formation of the lens. The external wall
of the eye vesicle in its inferior part then begins to invaginate, as a
result of which the eye vesicle becomes transformed into the eye cup
(Pig. 3). Its internal layer is the rudiment of the retina, while its external
layer is that of the pigment epithelium.
At first the internal layer is but slightly thicker than the external,
but later it thickens considerably. The external layer, on the contrary,
becomes single layered and, accumulating melanin granules, transforms
into the pigment epithelium. At this stage the internal layer is closely
adjacent to the lens rudiment. Invagination involves not only the eye
vesicle, but the eye stalk as well; the cavity of the eye cup (secondary
eye cavity) remains open for some time, due to the preservation of the
choroid fissure on the lower side of the eye (Fig. 4). Along this fissure the
hyaloid artery (when present) grows into the eye; then the lips of the
choroid fissure fuse, and the secondary eye cavity becomes closed.
Exact investigations of the developmental capacities of parts of the
primary eye rudiment began with Dragomirov (1932, 1933a, 1935).
Transplanting pieces of presumptive pigment epithelium and of presumptive retina of Anurans and Urodeles at the stage of the eye vesicle
and eye cup to various parts of the head of the embryo, he showed that
both layers gave rise to both the retina and the pigment epithelium,
which are sometimes organized into small eye cups. A similar mutual
reversibility of layers was shown in chicks up to the stage of 36 somites
when cultivating eye rudiments in vitro and on the chorioallantois
G.
V. LOPASHOV AND O. G.
STROEVA
from which the forebrain and eye rudiment are later formed, between
anuran species of which the embryos and their eyes differ considerably
in size (Balinsky, 1958).
The marking out of eye rudiments within the neural plate leads later
to their separation from the brain; these take part in this separation the
phenomena of negative affinity which arises between the material of the
eye rudiment and that of the remaining brain (Holtfreter, 1939). These
processes, however, do not provide for the further normal formation of
the eye rudiments; other conditions are required for the achievement of
the typical structure of the eye. These conditions act during the following
stage of development, and will be discussed in the next section.
III. Mechanisms of Segregation and Organization
of the Primary Eye Rudiment
By the end of neurulation the eye rudiments transform into vesicles,
paired outgrowths of the forebrain. In most Vertebrates they are hollow,
their cavity being connected with that of the brain through the eye
stalk. Increasing in size, eye vesicles come into contact with the ectoderm, in which they induce the formation of the lens. The external wall
of the eye vesicle in its inferior part then begins to invaginate, as a
result of which the eye vesicle becomes transformed into the eye cup
(Pig. 3). Its internal layer is the rudiment of the retina, while its external
layer is that of the pigment epithelium.
At first the internal layer is but slightly thicker than the external,
but later it thickens considerably. The external layer, on the contrary,
becomes single layered and, accumulating melanin granules, transforms
into the pigment epithelium. At this stage the internal layer is closely
adjacent to the lens rudiment. Invagination involves not only the eye
vesicle, but the eye stalk as well; the cavity of the eye cup (secondary
eye cavity) remains open for some time, due to the preservation of the
choroid fissure on the lower side of the eye (Fig. 4). Along this fissure the
hyaloid artery (when present) grows into the eye; then the lips of the
choroid fissure fuse, and the secondary eye cavity becomes closed.
Exact investigations of the developmental capacities of parts of the
primary eye rudiment began with Dragomirov (1932, 1933a, 1935).
Transplanting pieces of presumptive pigment epithelium and of presumptive retina of Anurans and Urodeles at the stage of the eye vesicle
and eye cup to various parts of the head of the embryo, he showed that
both layers gave rise to both the retina and the pigment epithelium,
which are sometimes organized into small eye cups. A similar mutual
reversibility of layers was shown in chicks up to the stage of 36 somites
when cultivating eye rudiments in vitro and on the chorioallantois