36
J. CLAVERT
the gastrula invaginates, the invagination occurs on the same side as the
crescents and in the vertical plane which passes through their centres.
Serial sections show that this is independent of the entrance point of the
spermatozoon. In fact invagination starts here even when the spermatozoon entered on this side. Vertical sections through the centre show that
the egg is deformed by the enforced position in which it has been kept.
The false grey crescent appears to be a region of the originally pigmented surface from which the pigment has moved to the opposite side
of the egg. A noticeable shift of the yolk from the dorsal side to the
ventral side has also taken place. The yolk of the cupola seems to have
flowed down to the ventral side, which is lowermost, under the influence
of gravity. Thus the ventral vitelline horn has spread towards the
animal pole, which it nearly attains, whereas the dorsal horn has become
more distant from this pole. The dorsal vitelline horn comes close
against the vitelline membrane. The effect of egg axis inclination in a
moist chamber can be nullified by subsequently inclining the egg in the
opposite direction. If this is done one hour after fertilization on eggs
already having a false crescent, the gastrula invaginates on the opposite
side to that of the false crescent. The second inclination has therefore
blotted out the first. The same results are obtained with activated
eggs.
To conclude: Inclining a fertilized or activated egg of Rana fusca in a
moist chamber orientates the plane of bilateral symmetry and nullifies the
action of the spermatozoon. This enforced position acts effectively upon
the determination of the plane of bilateral symmetry. Here again one
detects structural modifications of the egg which prefigure those
occurring during the rotation of fertilization.
c. Compression of the egg
Banki (1927), Weigmann (1926), Tung (1933), Pasteele (1946),
Ancel and Vintemberger (1948), Ancel and Calame (1959) have shown
that the fertilized or activated egg can be symmetrized by compression.
The means employed by these authors differ. Compression was obtained
either by placing an egg between two glass slides which were brought
together until the egg was clearly ovoid, or by squeezing the egg
slightly by a downward movement of a weighted glass plate supported
by plasticine. It is obvious that this compression must be applied before
the grey crescent is formed, and must be maintained until it appears.
If one has noticed beforehand the entrance point of the spermatozoon,
one can see whether the plane of symmetry obtained coincides with the
plane of fertilization. The authors generally conclude that compression
can determine the appearance of the grey crescent in a region different
from that in which it would have been localized by the spermatozoon:
J. CLAVERT
the gastrula invaginates, the invagination occurs on the same side as the
crescents and in the vertical plane which passes through their centres.
Serial sections show that this is independent of the entrance point of the
spermatozoon. In fact invagination starts here even when the spermatozoon entered on this side. Vertical sections through the centre show that
the egg is deformed by the enforced position in which it has been kept.
The false grey crescent appears to be a region of the originally pigmented surface from which the pigment has moved to the opposite side
of the egg. A noticeable shift of the yolk from the dorsal side to the
ventral side has also taken place. The yolk of the cupola seems to have
flowed down to the ventral side, which is lowermost, under the influence
of gravity. Thus the ventral vitelline horn has spread towards the
animal pole, which it nearly attains, whereas the dorsal horn has become
more distant from this pole. The dorsal vitelline horn comes close
against the vitelline membrane. The effect of egg axis inclination in a
moist chamber can be nullified by subsequently inclining the egg in the
opposite direction. If this is done one hour after fertilization on eggs
already having a false crescent, the gastrula invaginates on the opposite
side to that of the false crescent. The second inclination has therefore
blotted out the first. The same results are obtained with activated
eggs.
To conclude: Inclining a fertilized or activated egg of Rana fusca in a
moist chamber orientates the plane of bilateral symmetry and nullifies the
action of the spermatozoon. This enforced position acts effectively upon
the determination of the plane of bilateral symmetry. Here again one
detects structural modifications of the egg which prefigure those
occurring during the rotation of fertilization.
c. Compression of the egg
Banki (1927), Weigmann (1926), Tung (1933), Pasteele (1946),
Ancel and Vintemberger (1948), Ancel and Calame (1959) have shown
that the fertilized or activated egg can be symmetrized by compression.
The means employed by these authors differ. Compression was obtained
either by placing an egg between two glass slides which were brought
together until the egg was clearly ovoid, or by squeezing the egg
slightly by a downward movement of a weighted glass plate supported
by plasticine. It is obvious that this compression must be applied before
the grey crescent is formed, and must be maintained until it appears.
If one has noticed beforehand the entrance point of the spermatozoon,
one can see whether the plane of symmetry obtained coincides with the
plane of fertilization. The authors generally conclude that compression
can determine the appearance of the grey crescent in a region different
from that in which it would have been localized by the spermatozoon: