SYMMETRIZATION OF THE EGCJ OF VERTEBRATES 37
Ancel and Calame (1959) add, more precisely, that the plane of symmetry of eggs laterally compressed between two parallel plates is
always parallel to the compressing planes, and that the direction of the
dorsoventral axis depends upon the slant of the polar axis when the eggs
are first compressed.
Compression, therefore, like the directed rotation of orientation and
the enforced inclination of the egg, is only an experimental device to
determine the orientation of the plane of bilateral symmetry in Rana
usca. In natural conditions the entrance point of the spermatozoon is
always the determining factor. The authors who, following Weigmann
(1926), Banki (1927) and Tung (1933), have denied the determining
action of the spermatozoon on the plane of bilateral symmetry, have
experimented on compressed eggs, thus introducing unwittingly a source
of error.
B. Other Amphibians
1. Anurans {Rana esculenta, Discoglossus pictus, Bufo vulgaris)
In Rana esculenta, the unfertilized egg possesses a pigmented cap at
the animal pole, which is smaller than that of Rana fusca and sometimes off-centre (Roux, 1887; Schleip, 1929; Pasteels, 1937). I n this
species there is no true grey crescent, but the pigmented cap rises
noticeably on the dorsal side after fertilization. The whitish zone which
appears above the equator can be considered as the equivalent of the
grey crescent of Rana fusca or of Discoglossus, all the more so since on
the ventral side the black pigment descends and forms the Weigmann
striae. Pasteels (1937) has shown that in these eggs with an asymmetric
structure the dorsal region of the future embryo appears most frequently on the side where the pigment is highest. Ancel and Vintemberger (1948) found the same in activated eggs, but also demonstrated
that in normal conditions the spermatozoon is extremely important in
the determination of bilateral symmetry. They have shown that it is
also possible to orientate the plane of symmetry by rotations of
orientation. Wintrebert (1931) pointed out that in Discoglossus pictus
the clearly visible grey crescent that is formed is always in close relatipn
with the entrance point of the spermatozoon. He concludes that the
entrance point of the spermatozoon fixes the plane of bilateral symmetry of the future embryo. Ancel and Vintemberger (1948) have shown
that in Bufo vulgaris the spermatozoon determines the plane of bilateral
symmetry, that directed rotations of orientation may determine this
plane in activated eggs, and that these symmetrizing rotations can only
act before the grey crescent is formed.
These three anurans exhibit the essential features of determination
as evidenced in Rana fusca, but the studies are not so comprehensive.
Ancel and Calame (1959) add, more precisely, that the plane of symmetry of eggs laterally compressed between two parallel plates is
always parallel to the compressing planes, and that the direction of the
dorsoventral axis depends upon the slant of the polar axis when the eggs
are first compressed.
Compression, therefore, like the directed rotation of orientation and
the enforced inclination of the egg, is only an experimental device to
determine the orientation of the plane of bilateral symmetry in Rana
usca. In natural conditions the entrance point of the spermatozoon is
always the determining factor. The authors who, following Weigmann
(1926), Banki (1927) and Tung (1933), have denied the determining
action of the spermatozoon on the plane of bilateral symmetry, have
experimented on compressed eggs, thus introducing unwittingly a source
of error.
B. Other Amphibians
1. Anurans {Rana esculenta, Discoglossus pictus, Bufo vulgaris)
In Rana esculenta, the unfertilized egg possesses a pigmented cap at
the animal pole, which is smaller than that of Rana fusca and sometimes off-centre (Roux, 1887; Schleip, 1929; Pasteels, 1937). I n this
species there is no true grey crescent, but the pigmented cap rises
noticeably on the dorsal side after fertilization. The whitish zone which
appears above the equator can be considered as the equivalent of the
grey crescent of Rana fusca or of Discoglossus, all the more so since on
the ventral side the black pigment descends and forms the Weigmann
striae. Pasteels (1937) has shown that in these eggs with an asymmetric
structure the dorsal region of the future embryo appears most frequently on the side where the pigment is highest. Ancel and Vintemberger (1948) found the same in activated eggs, but also demonstrated
that in normal conditions the spermatozoon is extremely important in
the determination of bilateral symmetry. They have shown that it is
also possible to orientate the plane of symmetry by rotations of
orientation. Wintrebert (1931) pointed out that in Discoglossus pictus
the clearly visible grey crescent that is formed is always in close relatipn
with the entrance point of the spermatozoon. He concludes that the
entrance point of the spermatozoon fixes the plane of bilateral symmetry of the future embryo. Ancel and Vintemberger (1948) have shown
that in Bufo vulgaris the spermatozoon determines the plane of bilateral
symmetry, that directed rotations of orientation may determine this
plane in activated eggs, and that these symmetrizing rotations can only
act before the grey crescent is formed.
These three anurans exhibit the essential features of determination
as evidenced in Rana fusca, but the studies are not so comprehensive.