EGG CORTEX IN MORPHOGENESIS
365
first place, the role of the sperm entry has been considered, ascertained,
discussed, and sometimes denied. It is entirely to the credit of Ancel and
Vintemberger that they settled this question once and for all in their
definitive work published in 1948f.
From fertilization to first cleavage, two periods ought to be distinguished. The time when pronuclei copulate and when the grey (or clear)
crescent appears indicates the limit between the first period, which is
preparatory to symmetrization, and the second, when symmetry is
established.
1. The First Period
During the first period some symmetrical pattern may arise but
always in a sketchy and reversible way; this is a labile 'preparation'
towards bilateral symmetry.
The cortex is not yet affected by this preparation, but only the inner
structure. Several factors may act in the same way ; but if the two of
them act one after the other, and in different orientations, the second
may, if sufficiently potent, cancel the effects of the first, and impose its
own direction. The factors are: (1) the point of sperm entry; (2) a free
rotation of the egg inside its membrane; (3) an oblique position of the
egg with no possibility of rotation of the whole, and (4) a compression
of the egg.
The first of these factors, sperm entry, is the only one to act under
normal, physiological conditions. I shall have more to say later about
the third factor (oblique position of the egg, see § 3) ; and I am stating
here only that it acts always as a 'strong' factor, always obliterating the
previous effects of others, if they were differently directed.
The free rotation of the egg inside its chorion has been skilfully
explored by Ancel and Vintemberger (1948) : not only does its ability to
cancel the effects of a previous action depend upon the extent of the
rotation, but also upon the greater or lesser braking of this rotation,
which may be affected by a greater or lesser viscosity of the perivitelline
fluid. When made more difficult, the rotation has a more powerful effect.
It should be added that none of these factors is indispensable for the
realization of bilateral symmetry: an artifically activated egg, which
remains upright and thus never rotates, will nevertheless become
symmetrical in all cases. This implies some preformed symmetry of the
unfertilized egg as A. Brächet, in 1904, already concluded. Such a
preformed bilateral organization has been found, in fact, in the egg of
t From 1932 to 1948, Ancel and Vintemberger published no fewer than 47 preliminary
notes on parts of the subject, but all of them related only to the facts, without any
attempt at interpretation. To understand correctly the meaning of their conclusions,
consultation of their definitive work of 1948 is absolutely necessary.
365
first place, the role of the sperm entry has been considered, ascertained,
discussed, and sometimes denied. It is entirely to the credit of Ancel and
Vintemberger that they settled this question once and for all in their
definitive work published in 1948f.
From fertilization to first cleavage, two periods ought to be distinguished. The time when pronuclei copulate and when the grey (or clear)
crescent appears indicates the limit between the first period, which is
preparatory to symmetrization, and the second, when symmetry is
established.
1. The First Period
During the first period some symmetrical pattern may arise but
always in a sketchy and reversible way; this is a labile 'preparation'
towards bilateral symmetry.
The cortex is not yet affected by this preparation, but only the inner
structure. Several factors may act in the same way ; but if the two of
them act one after the other, and in different orientations, the second
may, if sufficiently potent, cancel the effects of the first, and impose its
own direction. The factors are: (1) the point of sperm entry; (2) a free
rotation of the egg inside its membrane; (3) an oblique position of the
egg with no possibility of rotation of the whole, and (4) a compression
of the egg.
The first of these factors, sperm entry, is the only one to act under
normal, physiological conditions. I shall have more to say later about
the third factor (oblique position of the egg, see § 3) ; and I am stating
here only that it acts always as a 'strong' factor, always obliterating the
previous effects of others, if they were differently directed.
The free rotation of the egg inside its chorion has been skilfully
explored by Ancel and Vintemberger (1948) : not only does its ability to
cancel the effects of a previous action depend upon the extent of the
rotation, but also upon the greater or lesser braking of this rotation,
which may be affected by a greater or lesser viscosity of the perivitelline
fluid. When made more difficult, the rotation has a more powerful effect.
It should be added that none of these factors is indispensable for the
realization of bilateral symmetry: an artifically activated egg, which
remains upright and thus never rotates, will nevertheless become
symmetrical in all cases. This implies some preformed symmetry of the
unfertilized egg as A. Brächet, in 1904, already concluded. Such a
preformed bilateral organization has been found, in fact, in the egg of
t From 1932 to 1948, Ancel and Vintemberger published no fewer than 47 preliminary
notes on parts of the subject, but all of them related only to the facts, without any
attempt at interpretation. To understand correctly the meaning of their conclusions,
consultation of their definitive work of 1948 is absolutely necessary.