SYMMETRIZATION OF THE EGG OF VERTEBRATES 55
one can obtain the inversion of the position of the embryogenic edge
until approximately the end of segmentation, as Wintrebert (1922)
found in selachians. Here again an external factor provokes egg symmetrization. Fautrez and Vakaet (1954), from morphological observations, described the transformation of a pre-existing plane of bilateral
symmetry in the oocyte into a final plane of symmetry in the embryo.
We were unable to repeat these observations on the same material
(Clavert and Filogamo, 1959) and our view is that in Lebistes reticulatus
the plane of bilateral symmetry is determined by the position of the
blastodisc on the egg surface at the time when the subgerminal cavity
appears. Many points however remain to be studied.
V I . Discussion and Interpretation of Results
We have seen that in the anurans the entrance point of the spermatozoon, with the animal-vegetal axis of polarity, determined the plane of
bilateral symmetry in the egg. Experimentally one can orientate the
plane of symmetry by other methods. Thus it has been established
(Ancel and Vintemberger, 1948) that the plane of symmetry is fixed once
and for all in normal conditions only when the grey crescent appears; the
spermatozoon, directed rotations of orientation, an enforced position,
or compression of the egg are only directive; these factors, all external,
orientate the direction of the future reaction of fertilization. Their
action is reversible, the action of one of these factors annihilating that of
a preceding factor. Their action is ineffective once the rotation of
fertilization has begun, so that in practice it is the last factor acting
before the grey crescent has begun to form during the rotation of
fertilization, which directs the orientation of the future plane of
bilateral symmetry. All these factors which direct the rotation of fertilization bring about reversible modifications in the egg, which start
off the irreversible transformations which take place during the
rotation of fertilization. These transformations consist of a displacement
of the cortex on the deeper layers, bringing about a medio-dorsal
concentration, which provokes a loss of pigment in a certain region
(hence the formation of the clear crescent). During this displacement
the cortex drags along part of the yolk which is in contact with it, thus
changing the relation between it and the deeper layers. This reaction,
as Dollander (1960), and we ourselves think, is apparently due to
important physico-chemical transformations of the cortex that have
not been defined yet (modification of the state or distribution of the
proteins, etc.). Dollander (1956) has shown that this hypothesis is
plausible by demonstrating ultra-structural modifications of the cortex.
This reaction of the egg 'itself which determines bilateral symmetry
one can obtain the inversion of the position of the embryogenic edge
until approximately the end of segmentation, as Wintrebert (1922)
found in selachians. Here again an external factor provokes egg symmetrization. Fautrez and Vakaet (1954), from morphological observations, described the transformation of a pre-existing plane of bilateral
symmetry in the oocyte into a final plane of symmetry in the embryo.
We were unable to repeat these observations on the same material
(Clavert and Filogamo, 1959) and our view is that in Lebistes reticulatus
the plane of bilateral symmetry is determined by the position of the
blastodisc on the egg surface at the time when the subgerminal cavity
appears. Many points however remain to be studied.
V I . Discussion and Interpretation of Results
We have seen that in the anurans the entrance point of the spermatozoon, with the animal-vegetal axis of polarity, determined the plane of
bilateral symmetry in the egg. Experimentally one can orientate the
plane of symmetry by other methods. Thus it has been established
(Ancel and Vintemberger, 1948) that the plane of symmetry is fixed once
and for all in normal conditions only when the grey crescent appears; the
spermatozoon, directed rotations of orientation, an enforced position,
or compression of the egg are only directive; these factors, all external,
orientate the direction of the future reaction of fertilization. Their
action is reversible, the action of one of these factors annihilating that of
a preceding factor. Their action is ineffective once the rotation of
fertilization has begun, so that in practice it is the last factor acting
before the grey crescent has begun to form during the rotation of
fertilization, which directs the orientation of the future plane of
bilateral symmetry. All these factors which direct the rotation of fertilization bring about reversible modifications in the egg, which start
off the irreversible transformations which take place during the
rotation of fertilization. These transformations consist of a displacement
of the cortex on the deeper layers, bringing about a medio-dorsal
concentration, which provokes a loss of pigment in a certain region
(hence the formation of the clear crescent). During this displacement
the cortex drags along part of the yolk which is in contact with it, thus
changing the relation between it and the deeper layers. This reaction,
as Dollander (1960), and we ourselves think, is apparently due to
important physico-chemical transformations of the cortex that have
not been defined yet (modification of the state or distribution of the
proteins, etc.). Dollander (1956) has shown that this hypothesis is
plausible by demonstrating ultra-structural modifications of the cortex.
This reaction of the egg 'itself which determines bilateral symmetry