SYMMETRIZATION OF THE EGG OF VERTEBRATES 35
sections made through the axis of the presumed future plane of bilateral
symmetry, these authors found that the cortex pigment descended
further on one side than on the other side, without, however, reaching
the inferior pole. The side where it was highest was the superior side
when the egg was so inclined as to permit gravity to determine its
rotation; according to preceding experiments we know that this will be
the future dorsal side. The pigment after the rotation of orientation has
ascended on the dorsal side. The dorsal horn of the vitelline cupola
touches the membrane, whereas on the ventral side it is separated from
the latter by a protoplasmic wedge. This dorsal horn is lower than the
ventral horn. The pigmentary movements and the modifications of the
connections and of the shape of the vitelline cupola bear witness to an
upheaval of the egg structure similar to that mentioned during the
rotation of fertilization. Only the amplitude differs; it is less in the case
of fertilization. We may note that the rotation of the membranes about
the egg has the same symmetrizing action as the rotation of the egg
within its membrane (Vintemberger and Clavert).
To conclude, a directed rotation of orientation, if its amplitude is
sufficient, determines the position of the grey crescent in the egg of
Ranafusca and thus in every case determines the position of the plane of
bilateral symmetry. The plane perpendicular to the axis of rotation and
passing through the axis of polarity of the egg becomes its plane of
bilateral symmetry. By means of directed rotations of orientation,
therefore, the egg of this amphibian can be 'symmetrized'. The modifications of structure in the egg act as a primer to those occurring in the
rotation of fertilization.
b. Maintenance of the fertilized egg in an inclined position
Ancel and Vintemberger (1948) have shown clearly the effect of an
inclined position imposed upon the egg axis on the determination of the
plane of bilateral symmetry in Rana fusca. Eggs are placed on slides
with the vitelline pole upwards, in such a way that their axes are turned
through 135°; after fertilization localized on the superior side, the eggs
are dipped briefly in water, and then placed in a moist chamber.
An hour after fertilization, false grey crescents can be observed clearly
in | of the eggs on their uppermost sides. The rotation of fertilization
then takes place, always in the vertical plane passing through the
centre of the false grey crescent, and carries the superior pole into the
hemisphere opposite to that containing the false crescent. When this
rotation is ended a grey area of varying length can be seen above the
false crescent extending it upwards, and in some cases reaching the
superior pole. This area is the true grey crescent which the rotation of
fertilization has determined. The future development shows that, when
sections made through the axis of the presumed future plane of bilateral
symmetry, these authors found that the cortex pigment descended
further on one side than on the other side, without, however, reaching
the inferior pole. The side where it was highest was the superior side
when the egg was so inclined as to permit gravity to determine its
rotation; according to preceding experiments we know that this will be
the future dorsal side. The pigment after the rotation of orientation has
ascended on the dorsal side. The dorsal horn of the vitelline cupola
touches the membrane, whereas on the ventral side it is separated from
the latter by a protoplasmic wedge. This dorsal horn is lower than the
ventral horn. The pigmentary movements and the modifications of the
connections and of the shape of the vitelline cupola bear witness to an
upheaval of the egg structure similar to that mentioned during the
rotation of fertilization. Only the amplitude differs; it is less in the case
of fertilization. We may note that the rotation of the membranes about
the egg has the same symmetrizing action as the rotation of the egg
within its membrane (Vintemberger and Clavert).
To conclude, a directed rotation of orientation, if its amplitude is
sufficient, determines the position of the grey crescent in the egg of
Ranafusca and thus in every case determines the position of the plane of
bilateral symmetry. The plane perpendicular to the axis of rotation and
passing through the axis of polarity of the egg becomes its plane of
bilateral symmetry. By means of directed rotations of orientation,
therefore, the egg of this amphibian can be 'symmetrized'. The modifications of structure in the egg act as a primer to those occurring in the
rotation of fertilization.
b. Maintenance of the fertilized egg in an inclined position
Ancel and Vintemberger (1948) have shown clearly the effect of an
inclined position imposed upon the egg axis on the determination of the
plane of bilateral symmetry in Rana fusca. Eggs are placed on slides
with the vitelline pole upwards, in such a way that their axes are turned
through 135°; after fertilization localized on the superior side, the eggs
are dipped briefly in water, and then placed in a moist chamber.
An hour after fertilization, false grey crescents can be observed clearly
in | of the eggs on their uppermost sides. The rotation of fertilization
then takes place, always in the vertical plane passing through the
centre of the false grey crescent, and carries the superior pole into the
hemisphere opposite to that containing the false crescent. When this
rotation is ended a grey area of varying length can be seen above the
false crescent extending it upwards, and in some cases reaching the
superior pole. This area is the true grey crescent which the rotation of
fertilization has determined. The future development shows that, when