SYMMETRIZATION OF THE EGG OF VERTEBRATES 51
relate this initial rotation to that observed in the hen during the
passage through the albumen-secreting part of the oviduct. We were
able to show (Clavert, 1960c) that the germ, which under normal
conditions is always found at the top of the egg under the mesometrium
in Lacerta stirpium, Tropidonotus natrix and Anguis fragilis (Clavert and
Zahnd, 1955; Ancel and Raynaud, 1958), is in the reverse position when
the animal is made to remain on its back for a long period. When an
egg of Lacerta stirpium is in the uterus, and a tracing mark is made with
carbon on the uterine wall, the membranes, and the yolk, the mark is
distorted in such a way that it can be explained only by rotation of
the outer layers of the egg within its membranes and by rotation of
the membranes within the uterus. We were unable to state precisely the
time when symmetrization occurs. Raynaud (1960) suggests that rotation of the membranes is compensated by a less rapid rotation of the
egg in the opposite direction in an attempt to attain equilibrium,
with the germ under the mesometrium; this is equally possible. Ancel
and Raynaud point out that 80% of the blastodiscs are oval in shape and
have their major axis parallel to the major axis of the egg. They consider that the minor axis of the oval blastodisc represents the future
plane of bilateral symmetry which is already determined at this stage.
There is no experimental confirmation of these morphological observations. The blastodisc may be deformed with the egg during the initial
rotation.
To sum up, while all authors agree that in reptiles the rule of von
Baer can be applied to the orientation of the embryo and that the
rotation of the egg in the uterus determines the orientation of the
embryonic axis, the moment when the plane of symmetry is fixed
remains uncertain. Further experiments are necessary to solve this
problem.
V. Determination of Bilateral Symmetry in Fish
The study of the phenomena of symmetrization in the eggs of fish
has given extremely variable results according to species. In the
following survey of the results obtained, three major types are distinguished—the Chondrostei, the Selachii and the Teleostei. Other
groups have not yet been specially studied.
A. Chondrostei (Acipenser sturio, A. ruthenus, A. huso)
The determination of the plane of bilateral symmetry was studied by
Dettlaff and Ginsburg (1953). These authors clearly showed the action
of an external factor. The egg of the sturgeon has the same structure as
that of the amphibians. It has, however, more yolk and it is pigmented.
relate this initial rotation to that observed in the hen during the
passage through the albumen-secreting part of the oviduct. We were
able to show (Clavert, 1960c) that the germ, which under normal
conditions is always found at the top of the egg under the mesometrium
in Lacerta stirpium, Tropidonotus natrix and Anguis fragilis (Clavert and
Zahnd, 1955; Ancel and Raynaud, 1958), is in the reverse position when
the animal is made to remain on its back for a long period. When an
egg of Lacerta stirpium is in the uterus, and a tracing mark is made with
carbon on the uterine wall, the membranes, and the yolk, the mark is
distorted in such a way that it can be explained only by rotation of
the outer layers of the egg within its membranes and by rotation of
the membranes within the uterus. We were unable to state precisely the
time when symmetrization occurs. Raynaud (1960) suggests that rotation of the membranes is compensated by a less rapid rotation of the
egg in the opposite direction in an attempt to attain equilibrium,
with the germ under the mesometrium; this is equally possible. Ancel
and Raynaud point out that 80% of the blastodiscs are oval in shape and
have their major axis parallel to the major axis of the egg. They consider that the minor axis of the oval blastodisc represents the future
plane of bilateral symmetry which is already determined at this stage.
There is no experimental confirmation of these morphological observations. The blastodisc may be deformed with the egg during the initial
rotation.
To sum up, while all authors agree that in reptiles the rule of von
Baer can be applied to the orientation of the embryo and that the
rotation of the egg in the uterus determines the orientation of the
embryonic axis, the moment when the plane of symmetry is fixed
remains uncertain. Further experiments are necessary to solve this
problem.
V. Determination of Bilateral Symmetry in Fish
The study of the phenomena of symmetrization in the eggs of fish
has given extremely variable results according to species. In the
following survey of the results obtained, three major types are distinguished—the Chondrostei, the Selachii and the Teleostei. Other
groups have not yet been specially studied.
A. Chondrostei (Acipenser sturio, A. ruthenus, A. huso)
The determination of the plane of bilateral symmetry was studied by
Dettlaff and Ginsburg (1953). These authors clearly showed the action
of an external factor. The egg of the sturgeon has the same structure as
that of the amphibians. It has, however, more yolk and it is pigmented.