DEVELOPMENT OF GASTROPODS
21
second maturation division and at cleavage at the original vegetal pole,
irrespective of the egg substances lying in this region. Hence, the lobe
may contain oil, hyaloplasm or yolk, according to the circumstances
(Morgan, 1933,1935a, b) (Fig. 7).
We may conclude from these experiments that the original polarity
of the eggs is not as a rule changed, at least by moderate centrifuging ;
presumably, it is bound to the cortex. The polar migration of the
maturation spindle and the position of cleavage spindles in normal
development are controlled by factors residing in the cortex.
Apparently, only by very strong ultra-centrifugations, with centrifugal forces up to more than 100,000
which have, according to other
data, a softening effect on the cortex, is it sometimes possible to deform
the cortical field, and thus to cause deviations in development. Costello
(1939) and Peltrera (1940) claim by this means to have altered the
original polarity of the egg. In Costello's experiments many abnormal
veligers developed from Diaulula eggs ultra-centrifuged at the uncleaved
stage. Peltrera got a number of double monsters from Aplysia
eggs
centrifuged at the 2- or 4-cell stage.
The view that the cortex controls the positions of the nuclei and
spindles is confirmed by the fact that the latter may be influenced by
exposing the eggs to external factors affecting the cortex. In Limnaea
eggs treated with weakly hypertonic solutions of CaCl 2 (Raven and
Mighorst, 1946) or LiCl (de Groot, 1948) the second maturation spindle
may orient itself at right angles to the egg axis, and a 'submerged'
second maturation division may ensue. In other cases, giant polar
bodies are formed. Sometimes, the egg karyomeres appearing after
extrusion of the second polar body are displaced towards the egg centre,
and the migration of sperm aster and sperm nucleus is suppressed or
delayed. Lithium chloride may also cause deviations in the direction of
cleavage spindles in Limnaea (Raven and Roborgh, 1949). Moreover, a
treatment of the eggs with LiCl solutions during third cleavage provokes
a reduction in size of the micromeres, presumably owing to a displacement of the spindles towards the animal pole (Raven et al., 1952). In
Limnaea eggs treated with LiCl at the 24-cell stage the nuclei, at least in
the animal cells, are displaced towards the surface (Raven and Dudok de
Wit, 1949).
Summarizing, it appears that both the factors controlling ooplasmic
segregation and those responsible for the localization and direction of
nuclei and spindles, and thus for the place and direction of cleavage
furrows, are somehow or other bound to the cortex, i.e., as we have seen,
presumably to the outer plasma membrane of the egg. As the typical
course of development is greatly dependent on the normal distribution
of substances among the cleavage cells, and therefore on an orderly
Β
21
second maturation division and at cleavage at the original vegetal pole,
irrespective of the egg substances lying in this region. Hence, the lobe
may contain oil, hyaloplasm or yolk, according to the circumstances
(Morgan, 1933,1935a, b) (Fig. 7).
We may conclude from these experiments that the original polarity
of the eggs is not as a rule changed, at least by moderate centrifuging ;
presumably, it is bound to the cortex. The polar migration of the
maturation spindle and the position of cleavage spindles in normal
development are controlled by factors residing in the cortex.
Apparently, only by very strong ultra-centrifugations, with centrifugal forces up to more than 100,000
which have, according to other
data, a softening effect on the cortex, is it sometimes possible to deform
the cortical field, and thus to cause deviations in development. Costello
(1939) and Peltrera (1940) claim by this means to have altered the
original polarity of the egg. In Costello's experiments many abnormal
veligers developed from Diaulula eggs ultra-centrifuged at the uncleaved
stage. Peltrera got a number of double monsters from Aplysia
eggs
centrifuged at the 2- or 4-cell stage.
The view that the cortex controls the positions of the nuclei and
spindles is confirmed by the fact that the latter may be influenced by
exposing the eggs to external factors affecting the cortex. In Limnaea
eggs treated with weakly hypertonic solutions of CaCl 2 (Raven and
Mighorst, 1946) or LiCl (de Groot, 1948) the second maturation spindle
may orient itself at right angles to the egg axis, and a 'submerged'
second maturation division may ensue. In other cases, giant polar
bodies are formed. Sometimes, the egg karyomeres appearing after
extrusion of the second polar body are displaced towards the egg centre,
and the migration of sperm aster and sperm nucleus is suppressed or
delayed. Lithium chloride may also cause deviations in the direction of
cleavage spindles in Limnaea (Raven and Roborgh, 1949). Moreover, a
treatment of the eggs with LiCl solutions during third cleavage provokes
a reduction in size of the micromeres, presumably owing to a displacement of the spindles towards the animal pole (Raven et al., 1952). In
Limnaea eggs treated with LiCl at the 24-cell stage the nuclei, at least in
the animal cells, are displaced towards the surface (Raven and Dudok de
Wit, 1949).
Summarizing, it appears that both the factors controlling ooplasmic
segregation and those responsible for the localization and direction of
nuclei and spindles, and thus for the place and direction of cleavage
furrows, are somehow or other bound to the cortex, i.e., as we have seen,
presumably to the outer plasma membrane of the egg. As the typical
course of development is greatly dependent on the normal distribution
of substances among the cleavage cells, and therefore on an orderly
Β