38
J. CLAVERT
In these species the amplitude of the rotation of fertilization which
definitely fixes the plane of bilateral symmetry is always 30°.
2. Urodeles (Triturus and Ambystoma mexicanum)
The eggs of Triturus spp. and Ambystoma show a clear crescent
corresponding to the grey crescent of the anurans before the first
cleavage appears, but not as neatly defined. The rotation of fertilization
which determines its formation is less, 15° as opposed to 30° in the
anurans.
Dollander (1956) has shown that the cortex of the symmetrized egg
of Triturus becomes modified: the villi present at the periphery in the
non-symmetrized egg disappear, and a condensed zone appears under
the limiting membrane of the egg. Yet it is impossible to ascertain the
nature of the transformations which have taken place (physicochemical alteration of cortical proteins, transformation of the macromolecular organization of the peripheral layer, or something else?).
The smallness, the lack of clear definition, the fact that it is difficult
to observe the invagination of the gastrula because of the thickness of
the jelly membrane, the extreme mobility of the egg in its jelly membrane, and the normal polyspermia, make it all the more difficult to
study symmetrization in the eggs of Triturus species. We are not therefore well informed about it. Ancel and Vintemberger (1948) studied
localized fertilization in Triturus alpestris and reached the conclusion
that the plane of bilateral symmetry can be determined in the ovulated
eggs by both or either of the factors (spermatozoon or rotation of
orientation) that determine it in anurans.
The eggs of urodeles therefore, like those of the anurans, have a
rotation of fertilization, but lack of experimentation and the normal
polyspermia make it impossible to be certain of the factor or factors
which determine bilateral symmetry in natural conditions.
C. Conclusions
In natural conditions the spermatozoon determines the plane of
bilateral symmetry of the future embryo in anurans. The sperm track
can always be found in sections made through the plane of bilateral
symmetry after the formation of the grey crescent.
It is possible to determine the plane of bilateral symmetry in all
amphibians by various experimental means such as by directed rotations
of orientation, provided they have sufficient amplitude; by keeping the
egg axis inclined; or by compressing the egg.
All these experimental devices, like the spermatozoon itself, modify
the structure and organization of the fertilized egg. The modification
thus provoked represents a forerunner of that which occurs during the
J. CLAVERT
In these species the amplitude of the rotation of fertilization which
definitely fixes the plane of bilateral symmetry is always 30°.
2. Urodeles (Triturus and Ambystoma mexicanum)
The eggs of Triturus spp. and Ambystoma show a clear crescent
corresponding to the grey crescent of the anurans before the first
cleavage appears, but not as neatly defined. The rotation of fertilization
which determines its formation is less, 15° as opposed to 30° in the
anurans.
Dollander (1956) has shown that the cortex of the symmetrized egg
of Triturus becomes modified: the villi present at the periphery in the
non-symmetrized egg disappear, and a condensed zone appears under
the limiting membrane of the egg. Yet it is impossible to ascertain the
nature of the transformations which have taken place (physicochemical alteration of cortical proteins, transformation of the macromolecular organization of the peripheral layer, or something else?).
The smallness, the lack of clear definition, the fact that it is difficult
to observe the invagination of the gastrula because of the thickness of
the jelly membrane, the extreme mobility of the egg in its jelly membrane, and the normal polyspermia, make it all the more difficult to
study symmetrization in the eggs of Triturus species. We are not therefore well informed about it. Ancel and Vintemberger (1948) studied
localized fertilization in Triturus alpestris and reached the conclusion
that the plane of bilateral symmetry can be determined in the ovulated
eggs by both or either of the factors (spermatozoon or rotation of
orientation) that determine it in anurans.
The eggs of urodeles therefore, like those of the anurans, have a
rotation of fertilization, but lack of experimentation and the normal
polyspermia make it impossible to be certain of the factor or factors
which determine bilateral symmetry in natural conditions.
C. Conclusions
In natural conditions the spermatozoon determines the plane of
bilateral symmetry of the future embryo in anurans. The sperm track
can always be found in sections made through the plane of bilateral
symmetry after the formation of the grey crescent.
It is possible to determine the plane of bilateral symmetry in all
amphibians by various experimental means such as by directed rotations
of orientation, provided they have sufficient amplitude; by keeping the
egg axis inclined; or by compressing the egg.
All these experimental devices, like the spermatozoon itself, modify
the structure and organization of the fertilized egg. The modification
thus provoked represents a forerunner of that which occurs during the