SYMMETRIZATION OF THE EGG OF VERTEBRATES 53
shown that this rotation of orientation through 90° normally fixes the
position of the clear crescent. The movement described by the animal
pole traces a curve which defines, with the axis of polarity of the egg,
the future plane of symmetry. As in amphibians, the effect of one
rotation can be nullified by a subsequent rotation, the last one always
being determining. Except for the nature of the preparatory external
factor, the determination of the plane of symmetry in the egg of
Chondrostei takes place as in amphibians. We have seen that this
preparatory factor (rotation of orientation) can have the same effect on
symmetrization in the egg of the frog. Dettlaff and Ginsburg conclude
that there is a true analogy between the different phenomena of
symmetrization observed in amphibians and in the different species of
sturgeons.
One may infer from the development of the egg of lamprey and
Protopterus that they are similar in the determination of the plane of
symmetry to the Chondrostei.
B. Selachii (Scylliorhinus canicula)
The symmetrization of the egg of selachians has been studied by
Wintrebert (1922) in Scylliorhinus canicula. The structure of the egg of
Selachians resembles that of the avian egg, but there is a more massive
central part corresponding to the latebra, the neck of the latebra and
the nucleus of Pander. The egg is enveloped within similar membranes
to those of the avian egg, but within these membranes it is not so mobile
and the blastodisc is nearly always found in the superior hemisphere
very near to but not at the topmost point. The disc therefore usually
has a superior edge. During development a groove appears between the
blastodisc and the yolk. This groove always appears at the superior
edge. It is wider as the germ is more laterally situated. The groove
marks the future posterior edge of the blastodisc, the embryogenic
edge. Any part of the periphery of the blastodisc can give rise to this
embryogenic edge. The only condition is that it should correspond to
the highest point of the edge of the blastoderm. This determination can
take place as long as the subgerminal cavity is not apparent; when the
cavity is formed the position of the embryogenic edge, and therefore of
the plane of symmetry, is definitely fixed. Wintrebert concludes by
saying that no special region of the disc is destined to produce the
embryo. When segmentation ends, one can determine at will the region
where the embryogenic edge, and the subgerminal cavity, will appear,
by positioning it uppermost. Here again an external factor, the position
of the blastoderm in space, controls the position of the future plane
of bilateral symmetry of the embryo. Wintrebert does not however describe the specific reaction of the germ. Vivien and Hay (1954), experic
A.M. 2
shown that this rotation of orientation through 90° normally fixes the
position of the clear crescent. The movement described by the animal
pole traces a curve which defines, with the axis of polarity of the egg,
the future plane of symmetry. As in amphibians, the effect of one
rotation can be nullified by a subsequent rotation, the last one always
being determining. Except for the nature of the preparatory external
factor, the determination of the plane of symmetry in the egg of
Chondrostei takes place as in amphibians. We have seen that this
preparatory factor (rotation of orientation) can have the same effect on
symmetrization in the egg of the frog. Dettlaff and Ginsburg conclude
that there is a true analogy between the different phenomena of
symmetrization observed in amphibians and in the different species of
sturgeons.
One may infer from the development of the egg of lamprey and
Protopterus that they are similar in the determination of the plane of
symmetry to the Chondrostei.
B. Selachii (Scylliorhinus canicula)
The symmetrization of the egg of selachians has been studied by
Wintrebert (1922) in Scylliorhinus canicula. The structure of the egg of
Selachians resembles that of the avian egg, but there is a more massive
central part corresponding to the latebra, the neck of the latebra and
the nucleus of Pander. The egg is enveloped within similar membranes
to those of the avian egg, but within these membranes it is not so mobile
and the blastodisc is nearly always found in the superior hemisphere
very near to but not at the topmost point. The disc therefore usually
has a superior edge. During development a groove appears between the
blastodisc and the yolk. This groove always appears at the superior
edge. It is wider as the germ is more laterally situated. The groove
marks the future posterior edge of the blastodisc, the embryogenic
edge. Any part of the periphery of the blastodisc can give rise to this
embryogenic edge. The only condition is that it should correspond to
the highest point of the edge of the blastoderm. This determination can
take place as long as the subgerminal cavity is not apparent; when the
cavity is formed the position of the embryogenic edge, and therefore of
the plane of symmetry, is definitely fixed. Wintrebert concludes by
saying that no special region of the disc is destined to produce the
embryo. When segmentation ends, one can determine at will the region
where the embryogenic edge, and the subgerminal cavity, will appear,
by positioning it uppermost. Here again an external factor, the position
of the blastoderm in space, controls the position of the future plane
of bilateral symmetry of the embryo. Wintrebert does not however describe the specific reaction of the germ. Vivien and Hay (1954), experic
A.M. 2