14
CHR. P. RAVEN
ooplasmic segregation are bound to the egg cortex. By local attractions
and repulsions it may control the arrangement of substances in the more
fluid inner cytoplasm. For instance, we may assume that the cortex near
the animal pole in the Limnaea egg has special properties, as a result of
which it begins at a certain stage to attract or to capture particular
components of the cytoplasm.
This view receives further support from those cases where ooplasmic
segregation is disturbed by external agencies which may be supposed to
primarily affect the egg cortex. For instance, in eggs of Limnaea treated
with lithium chloride the normal distribution of the subcortical plasm
may be disturbed, and the formation of the animal pole plasm entirely
suppressed (de Groot, 1948). When the eggs are subsequently transferred
to tap-water, recovery may occur and a pole plasm form after some
delay (van den Broek and Raven, 1951).
V. Nature, Composition and Properties of the Cortex
We have seen that the egg cortex, in contrast to the more fluid internal
cytoplasm, has a marked rigidity. This cortical rigidity can be evaluated
by determining the forces necessary to elongate or pull apart the egg.
Its absolute value has been determined in this way by Harvey (1931) for
the egg of Ilyanassa,
where he found a value of 1-1 dynes per cm.
Fluctuations in cortical rigidity have been observed during the uncleaved
stage in Limnaea (Raven, 1945). It is low during the extrusion of the
first polar body, shortly after the extrusion of the second polar body,
and immediately before first cleavage, higher between these minima.
The first two of these minima coincide with periods of amoeboid
motility. It is evident that similar fluctuations in overall rigidity occur
during cleavage ; moreover, differences in rigidity among various parts
of the surface probably play a part in cell division. A decrease in cortical
rigidity has been observed in Ilyanassa by Butros (1956) after treatment
of the eggs with sodium azide, trichlorophenol or monoiodoacetic acid.
This is explained by assuming that a denaturation of proteins, accompanied by uncoiling and stretching of polypeptide chains,has taken place.
Cytochemical observations have shown that the cortex in the egg of
Limnaea is rich in sulphydryl compounds and in RNA. When the eggs
are stained according to the method of Unna-Brachet, the outer plasma
membrane is visible as a distinct deep-red line; beneath it there is a
narrow subcortical layer staining more deeply than the rest of the
cytoplasm. In centrifuged eggs this subcortical layer is almost restricted
to the zone of the hyaloplasm, but the deep-red outer lamella also
extends over the oil and yolk zones. After ribonuclease treatment the
whole egg remains colourless.
CHR. P. RAVEN
ooplasmic segregation are bound to the egg cortex. By local attractions
and repulsions it may control the arrangement of substances in the more
fluid inner cytoplasm. For instance, we may assume that the cortex near
the animal pole in the Limnaea egg has special properties, as a result of
which it begins at a certain stage to attract or to capture particular
components of the cytoplasm.
This view receives further support from those cases where ooplasmic
segregation is disturbed by external agencies which may be supposed to
primarily affect the egg cortex. For instance, in eggs of Limnaea treated
with lithium chloride the normal distribution of the subcortical plasm
may be disturbed, and the formation of the animal pole plasm entirely
suppressed (de Groot, 1948). When the eggs are subsequently transferred
to tap-water, recovery may occur and a pole plasm form after some
delay (van den Broek and Raven, 1951).
V. Nature, Composition and Properties of the Cortex
We have seen that the egg cortex, in contrast to the more fluid internal
cytoplasm, has a marked rigidity. This cortical rigidity can be evaluated
by determining the forces necessary to elongate or pull apart the egg.
Its absolute value has been determined in this way by Harvey (1931) for
the egg of Ilyanassa,
where he found a value of 1-1 dynes per cm.
Fluctuations in cortical rigidity have been observed during the uncleaved
stage in Limnaea (Raven, 1945). It is low during the extrusion of the
first polar body, shortly after the extrusion of the second polar body,
and immediately before first cleavage, higher between these minima.
The first two of these minima coincide with periods of amoeboid
motility. It is evident that similar fluctuations in overall rigidity occur
during cleavage ; moreover, differences in rigidity among various parts
of the surface probably play a part in cell division. A decrease in cortical
rigidity has been observed in Ilyanassa by Butros (1956) after treatment
of the eggs with sodium azide, trichlorophenol or monoiodoacetic acid.
This is explained by assuming that a denaturation of proteins, accompanied by uncoiling and stretching of polypeptide chains,has taken place.
Cytochemical observations have shown that the cortex in the egg of
Limnaea is rich in sulphydryl compounds and in RNA. When the eggs
are stained according to the method of Unna-Brachet, the outer plasma
membrane is visible as a distinct deep-red line; beneath it there is a
narrow subcortical layer staining more deeply than the rest of the
cytoplasm. In centrifuged eggs this subcortical layer is almost restricted
to the zone of the hyaloplasm, but the deep-red outer lamella also
extends over the oil and yolk zones. After ribonuclease treatment the
whole egg remains colourless.