DEVELOPMENT OF GASTROPODS
3
are endowed from the outset with a différent chemical composition of
their cytoplasm.
In the eggs of Limnaea stagnalis a special plasm accumulates at the
vegetal pole during their passage through the female genital duct
(Raven, 1945). This vegetal pole plasm is rich in protein yolk granules
(Fig. IB). In the recently laid egg it occupies a well-defined sector at the
vegetal pole, but soon it extends beneath the surface towards the animal
side. For some time the animal pole, where the two polar bodies are
extruded, is free from it, but then it spreads over this region also, forming
a continuous layer of nearly uniform thickness, the subcortical plasm,
around the egg.
A second cytoplasmic differentiation, occurring in various pulmonates
(Limnaea, Myxas, Planorbis, Physa, Succinea), is the animal pole plasm.
Immediately after oviposition it has not yet been formed, but it appears
during or shortly after the maturation divisions by the accumulation of
a special plasm beneath the egg cortex surrounding the animal pole.
This occurs at various moments in different species, e.g., prior to the
extrusion of the first polar body in Succinea putris (Jura, 1960) ; between
first and second maturation division in Limnaea palustris, L. ovata and
Myxas glutinosa ; some time after the extrusion of the second polar body
in Limnaea stagnalis (Raven, 1945). The animal pole plasm is very rich
in mitochondria. The latter are first concentrated around the maturation
spindles and asters. They are transported by this means towards the
animal pole, where they accumulate in dense layers immediately
beneath the cortex surrounding the animal pole.
At first and second cleavage, the cytoplasmic substances in Limnaea
are distributed about equally among the blastomeres. Prior to the third
cleavage, however, the subcortical plasm and animal pole plasm unite at
the animal pole into a common mass of dense cytoplasm. Most of this
plasm passes into the micromeres, which consequently consist for the
greater part of pole plasm substance, whereas the macromeres consist
mainly of vacuolated cytoplasm. This differential distribution of
cytoplasmic substances is repeated at the following cleavages, so that
the relative amount of pole plasm substance in the cells of the blastula
decreases from the animal towards the vegetal pole (Raven, 1946). It lies
in the superficial region in all cells. In older blastulae vacuoles appear in
this region, filled with egg capsule fluid, which is taken up by pinocytosis
(Elbers and Bluemink, 1960). A similar differential distribution of pole
plasm substance has also been found in Succinea (Jura, 1960).
Sometimes a vegetal pole plasm may temporarily be constricted off
from the rest of the egg, forming a so-called polar lobe. Such polar lobes
have been described in the eggs of various Prosobranchiata, e.g.,
Bithynia, Crepidula, Nassa, llyanassa,
Ocinebra, Urosalpinx,
Fulgur.
3
are endowed from the outset with a différent chemical composition of
their cytoplasm.
In the eggs of Limnaea stagnalis a special plasm accumulates at the
vegetal pole during their passage through the female genital duct
(Raven, 1945). This vegetal pole plasm is rich in protein yolk granules
(Fig. IB). In the recently laid egg it occupies a well-defined sector at the
vegetal pole, but soon it extends beneath the surface towards the animal
side. For some time the animal pole, where the two polar bodies are
extruded, is free from it, but then it spreads over this region also, forming
a continuous layer of nearly uniform thickness, the subcortical plasm,
around the egg.
A second cytoplasmic differentiation, occurring in various pulmonates
(Limnaea, Myxas, Planorbis, Physa, Succinea), is the animal pole plasm.
Immediately after oviposition it has not yet been formed, but it appears
during or shortly after the maturation divisions by the accumulation of
a special plasm beneath the egg cortex surrounding the animal pole.
This occurs at various moments in different species, e.g., prior to the
extrusion of the first polar body in Succinea putris (Jura, 1960) ; between
first and second maturation division in Limnaea palustris, L. ovata and
Myxas glutinosa ; some time after the extrusion of the second polar body
in Limnaea stagnalis (Raven, 1945). The animal pole plasm is very rich
in mitochondria. The latter are first concentrated around the maturation
spindles and asters. They are transported by this means towards the
animal pole, where they accumulate in dense layers immediately
beneath the cortex surrounding the animal pole.
At first and second cleavage, the cytoplasmic substances in Limnaea
are distributed about equally among the blastomeres. Prior to the third
cleavage, however, the subcortical plasm and animal pole plasm unite at
the animal pole into a common mass of dense cytoplasm. Most of this
plasm passes into the micromeres, which consequently consist for the
greater part of pole plasm substance, whereas the macromeres consist
mainly of vacuolated cytoplasm. This differential distribution of
cytoplasmic substances is repeated at the following cleavages, so that
the relative amount of pole plasm substance in the cells of the blastula
decreases from the animal towards the vegetal pole (Raven, 1946). It lies
in the superficial region in all cells. In older blastulae vacuoles appear in
this region, filled with egg capsule fluid, which is taken up by pinocytosis
(Elbers and Bluemink, 1960). A similar differential distribution of pole
plasm substance has also been found in Succinea (Jura, 1960).
Sometimes a vegetal pole plasm may temporarily be constricted off
from the rest of the egg, forming a so-called polar lobe. Such polar lobes
have been described in the eggs of various Prosobranchiata, e.g.,
Bithynia, Crepidula, Nassa, llyanassa,
Ocinebra, Urosalpinx,
Fulgur.