SYMMETRIZATION OF THE EGG OF VERTEBRATES 57
cephalo-caudal axis (which permits a thorough study of the symmetrization phenomena) and, as yet, we cannot state precisely how
bilateral symmetry is fixed in these eggs.
In fish, the work of Dettlaff and Ginsburg (1953, 1954) has shown
that in Chondrostei where the structure and development of the eggs
are similar, in principle, to those of amphibians, the plane of bilateral
symmetry is determined when the grey crescent appears under the
influence of an external factor, the rotation of orientation. As in
amphibians, the plane of symmetry is fixed early and segmentation is
total. In selachians and teleosts the plane of symmetry is fixed when the
subgerminal cavity begins to form; it is the position of the blastodisc on
the egg surface immediately beforehand which determines the position
of the embryogenic edge, and thus the position of the plane of bilateral
symmetry. Here, as in birds, the plane of bilateral symmetry is fixed
late, at the end of segmentation.
The findings of Tung, Chang, and Tung (1945) on the goldfish and
those of Devillers on the trout are difficult to interpret in the light of
our own results; for lack of sufficient evidence it is difficult to ascertain
whether or not, in these cases too, the position of the blastodisc in space
has an influence on egg symmetrization. These authors operated either
before or at the time when the plane of symmetry appears to be fixed
in the germ. Tung, Chang, and Tung claim the existence of a crescent
of symmetrization made of a hypothetical determining substance, and
fixed by early migration.
If this is so, how are we to explain that a late change alters the
position of the embryogenic edge, or even produces two embryogenic
edges, as shown in our experiments? (Clavert andFilogamo, 1957, 1959).
It will be necessary above all to isolate this substance so as to prove its
existence. Here again the influence of possible external factors cannot be
proved or described for lack of data.
To sum up, bilateral symmetry in fish is acquired in two different
ways: in Chondrostei, by a rotation of symmetrization which ends in
the formation of the grey crescent whose localization is fixed by the
rotation of orientation. This happens early, before segmentation starts.
In those teleosts and selachians that have been studied, the plane of
bilateral symmetry is determined by the position of the blastodisc in
space and the appearance of the subgerminal cavity. The effect of
position of the blastodisc in space can be compared to that of the
experimentally enforced inclination of the egg axis in amphibians, as in
this case also it is through the uppermost point of the marginal zone
that the plane of bilateral symmetry will pass.
The phenomena of symmetrization in the egg of Vertebrates offer
similarities and differences according to the type of egg. In all the case«
cephalo-caudal axis (which permits a thorough study of the symmetrization phenomena) and, as yet, we cannot state precisely how
bilateral symmetry is fixed in these eggs.
In fish, the work of Dettlaff and Ginsburg (1953, 1954) has shown
that in Chondrostei where the structure and development of the eggs
are similar, in principle, to those of amphibians, the plane of bilateral
symmetry is determined when the grey crescent appears under the
influence of an external factor, the rotation of orientation. As in
amphibians, the plane of symmetry is fixed early and segmentation is
total. In selachians and teleosts the plane of symmetry is fixed when the
subgerminal cavity begins to form; it is the position of the blastodisc on
the egg surface immediately beforehand which determines the position
of the embryogenic edge, and thus the position of the plane of bilateral
symmetry. Here, as in birds, the plane of bilateral symmetry is fixed
late, at the end of segmentation.
The findings of Tung, Chang, and Tung (1945) on the goldfish and
those of Devillers on the trout are difficult to interpret in the light of
our own results; for lack of sufficient evidence it is difficult to ascertain
whether or not, in these cases too, the position of the blastodisc in space
has an influence on egg symmetrization. These authors operated either
before or at the time when the plane of symmetry appears to be fixed
in the germ. Tung, Chang, and Tung claim the existence of a crescent
of symmetrization made of a hypothetical determining substance, and
fixed by early migration.
If this is so, how are we to explain that a late change alters the
position of the embryogenic edge, or even produces two embryogenic
edges, as shown in our experiments? (Clavert andFilogamo, 1957, 1959).
It will be necessary above all to isolate this substance so as to prove its
existence. Here again the influence of possible external factors cannot be
proved or described for lack of data.
To sum up, bilateral symmetry in fish is acquired in two different
ways: in Chondrostei, by a rotation of symmetrization which ends in
the formation of the grey crescent whose localization is fixed by the
rotation of orientation. This happens early, before segmentation starts.
In those teleosts and selachians that have been studied, the plane of
bilateral symmetry is determined by the position of the blastodisc in
space and the appearance of the subgerminal cavity. The effect of
position of the blastodisc in space can be compared to that of the
experimentally enforced inclination of the egg axis in amphibians, as in
this case also it is through the uppermost point of the marginal zone
that the plane of bilateral symmetry will pass.
The phenomena of symmetrization in the egg of Vertebrates offer
similarities and differences according to the type of egg. In all the case«