54
J. CLAVERT
menting on polyembryony, have shown that in selachians the axis of
the embryo is fixed at the beginning of gastrulation, thus confirming
indirectly the experiments of Wintrebert. The existence of a critical
period seems to indicate that in this selachian as in birds, the formation
of the subgerminal cavity is associated with a reaction of the germ
which at that particular moment fixes the plane of symmetry in a
direction determined by the last factor to have acted before the reaction
takes place.
C. Teleostei
Much work has been done on the symmetrization of the egg of
teleosts. Tung, Chang, and Tung (1945) in a study of Carassius auratus
were led to suggest the existence of a hypothetical crescent of symmetrization as a result of experiments involving latitudinal section.
FIG. 12. Diagram showing how the embryogenic edge is determined in Lebistes
reticulatus. The arrow indicates the embryogenic edge which is always formed at the
lower part of the blastodisc (after Clavert and Filogamo).
This crescent is made of a hypothetical determining substance, which
reaches its place by early migration. Devillers (1951) has also studied,
in the trout, the formation of the plane of bilateral symmetry, and
obtained interesting experimental results. He has shown that, in particular experimental conditions, the embryogenic edge still appears in
its normal place, as if this zone offered a special attraction during the
re-organization of the disc. The author thinks that the vitelline
syncytium of the egg may have a polarized structure capable of directing the symmetrization of the germ, at least at the beginning. Clavert
and Filogamo (1957, 1959) studied the formation of the embryogenic
edge in Lebistes reticulatus and showed that the position of the germ
in space determines the position of the embryogenic edge, for it always
appears on the lower edge of the blastodisc (Fig. 12). When its position
in space is modified by placing the fish on its back, the embryogenic
edge is formed at a point opposite to the normal. Here also it seems that
J. CLAVERT
menting on polyembryony, have shown that in selachians the axis of
the embryo is fixed at the beginning of gastrulation, thus confirming
indirectly the experiments of Wintrebert. The existence of a critical
period seems to indicate that in this selachian as in birds, the formation
of the subgerminal cavity is associated with a reaction of the germ
which at that particular moment fixes the plane of symmetry in a
direction determined by the last factor to have acted before the reaction
takes place.
C. Teleostei
Much work has been done on the symmetrization of the egg of
teleosts. Tung, Chang, and Tung (1945) in a study of Carassius auratus
were led to suggest the existence of a hypothetical crescent of symmetrization as a result of experiments involving latitudinal section.
FIG. 12. Diagram showing how the embryogenic edge is determined in Lebistes
reticulatus. The arrow indicates the embryogenic edge which is always formed at the
lower part of the blastodisc (after Clavert and Filogamo).
This crescent is made of a hypothetical determining substance, which
reaches its place by early migration. Devillers (1951) has also studied,
in the trout, the formation of the plane of bilateral symmetry, and
obtained interesting experimental results. He has shown that, in particular experimental conditions, the embryogenic edge still appears in
its normal place, as if this zone offered a special attraction during the
re-organization of the disc. The author thinks that the vitelline
syncytium of the egg may have a polarized structure capable of directing the symmetrization of the germ, at least at the beginning. Clavert
and Filogamo (1957, 1959) studied the formation of the embryogenic
edge in Lebistes reticulatus and showed that the position of the germ
in space determines the position of the embryogenic edge, for it always
appears on the lower edge of the blastodisc (Fig. 12). When its position
in space is modified by placing the fish on its back, the embryogenic
edge is formed at a point opposite to the normal. Here also it seems that