VIII.
MORPHOGENESIS OF THE VERTEBRATE
EYE
367
which establishes the retinal axes. Establishment of three axes proceeds
here in the same sequence as in the limb rudiment (Harrison, 1918;
Nicholas, 1955), and in the ear vesicle (Harrison, 1945; Yntema, 1955).
(c) Epithelio-mesenchymal interactions in cornea formation are
analogous to their relationships in the development of other organs
(limbs—Saunders, 1948; Zwilling, 1956; Tschumi, 1957; feather and
other skin derivatives—Sengel, 1958; Saunders, 1958; dorsal fins of
amphibian larvae—Stroeva, 1950a, b; mouse tail—Grüneberg, 1956). In
all these examples (independently of how it is actually realized in each
individual case), the mesodermal part determines the specificity of the
epithelium, while it is under the influence of the latter that the differentiation of the mesodermal portion of the organ rudiment occurs. Is the
character of the interaction between the mesenchyme and the pigment
epithelium not of the same character? The influence of the mesenchyme
on the differentiation of the pigment epithelium is well known (§111). The
character of the reverse relationship, the influence of the pigment
epithelium on the differentiation of the choroid coat, should be investigated.
Disturbance of individual links of development during the second
stage brings about multiple deviations from normal development.
Depending on the link affected, the direction of cell differentiation of
this or that portion of the rudiment can be altered and, accordingly, the
direction of a whole chain of causal relations is changed in its interaction
with other portions of the rudiment and surrounding tissues. It follows
from this, that the action (and character) of the external disturbing
agent has no direct connection with the results of its action. Such
complex deviations from the normal development can be best exemplified
by the colobomas of the retina and of the choroid coat of the eye.
At the same time, far reaching regulations are possible at this phase.
The processes leading to regulation undoubtedly must be closely
related to those of normal rudiment segregation. Beginning with
Driesch, the emergence of the organization of such rudiments and
similar systems were for a long time regarded as due to an inherent
capacity of the rudiments themselves. Such a conception for the eye
rudiment was evolved by Dragomirov (1933b) who considered it as a
gradient system in Child's sense, where either the rudiment or portions
of it segregate themselves to the dominant (retina, fibre nucleus of
the lens) and subordinate (pigment epithelium, lens epithelium) zones.
Further investigations proved this conception to be erroneous, as is
clear from the above.
3. Finally, after the formation of the main structure of the eye and
its component rudiments, there begins the third phase. Here, under the
influence of tangential forces of tension, arising as a result of the inter-
MORPHOGENESIS OF THE VERTEBRATE
EYE
367
which establishes the retinal axes. Establishment of three axes proceeds
here in the same sequence as in the limb rudiment (Harrison, 1918;
Nicholas, 1955), and in the ear vesicle (Harrison, 1945; Yntema, 1955).
(c) Epithelio-mesenchymal interactions in cornea formation are
analogous to their relationships in the development of other organs
(limbs—Saunders, 1948; Zwilling, 1956; Tschumi, 1957; feather and
other skin derivatives—Sengel, 1958; Saunders, 1958; dorsal fins of
amphibian larvae—Stroeva, 1950a, b; mouse tail—Grüneberg, 1956). In
all these examples (independently of how it is actually realized in each
individual case), the mesodermal part determines the specificity of the
epithelium, while it is under the influence of the latter that the differentiation of the mesodermal portion of the organ rudiment occurs. Is the
character of the interaction between the mesenchyme and the pigment
epithelium not of the same character? The influence of the mesenchyme
on the differentiation of the pigment epithelium is well known (§111). The
character of the reverse relationship, the influence of the pigment
epithelium on the differentiation of the choroid coat, should be investigated.
Disturbance of individual links of development during the second
stage brings about multiple deviations from normal development.
Depending on the link affected, the direction of cell differentiation of
this or that portion of the rudiment can be altered and, accordingly, the
direction of a whole chain of causal relations is changed in its interaction
with other portions of the rudiment and surrounding tissues. It follows
from this, that the action (and character) of the external disturbing
agent has no direct connection with the results of its action. Such
complex deviations from the normal development can be best exemplified
by the colobomas of the retina and of the choroid coat of the eye.
At the same time, far reaching regulations are possible at this phase.
The processes leading to regulation undoubtedly must be closely
related to those of normal rudiment segregation. Beginning with
Driesch, the emergence of the organization of such rudiments and
similar systems were for a long time regarded as due to an inherent
capacity of the rudiments themselves. Such a conception for the eye
rudiment was evolved by Dragomirov (1933b) who considered it as a
gradient system in Child's sense, where either the rudiment or portions
of it segregate themselves to the dominant (retina, fibre nucleus of
the lens) and subordinate (pigment epithelium, lens epithelium) zones.
Further investigations proved this conception to be erroneous, as is
clear from the above.
3. Finally, after the formation of the main structure of the eye and
its component rudiments, there begins the third phase. Here, under the
influence of tangential forces of tension, arising as a result of the inter-