52
J. CLAVERT
It has a micropyle situated exactly above the animal pole; fertilization
must take place at that level. The laid egg is somewhat oval so that it
always lies with its major axis parallel to the substrate. Immediately
after fertilization the envelope fixes the egg to the substrate. The egg
then becomes round and undergoes a rotation of orientation by which
the animal pole is brought to the highest point. A spontaneous rotation
FIG. 11. Three sturgeon eggs: (a) immediately after they have been laid; (b) after
the rotation of orientation which brings the animal pole (1) into the upper part; (c)
after the formation of the grey crescent. There is always a relation between the position
of the grey crescent (2) and the direction of the rotation of orientation (after Dettlaffand
Ginsburg, 1954).
of orientation through 90° has therefore taken place. At the beginning
of segmentation, which is total, a clear crescent appears. This clear
crescent results from a rotation of fertilization similar to that of amphibians, indicating an identical reaction of the egg itself (Fig. 11). The
appearance of the clear crescent indicates that the determination of the
plane of bilateral symmetry is final. Dettlaif and Ginsburg (1954) have
J. CLAVERT
It has a micropyle situated exactly above the animal pole; fertilization
must take place at that level. The laid egg is somewhat oval so that it
always lies with its major axis parallel to the substrate. Immediately
after fertilization the envelope fixes the egg to the substrate. The egg
then becomes round and undergoes a rotation of orientation by which
the animal pole is brought to the highest point. A spontaneous rotation
FIG. 11. Three sturgeon eggs: (a) immediately after they have been laid; (b) after
the rotation of orientation which brings the animal pole (1) into the upper part; (c)
after the formation of the grey crescent. There is always a relation between the position
of the grey crescent (2) and the direction of the rotation of orientation (after Dettlaffand
Ginsburg, 1954).
of orientation through 90° has therefore taken place. At the beginning
of segmentation, which is total, a clear crescent appears. This clear
crescent results from a rotation of fertilization similar to that of amphibians, indicating an identical reaction of the egg itself (Fig. 11). The
appearance of the clear crescent indicates that the determination of the
plane of bilateral symmetry is final. Dettlaif and Ginsburg (1954) have