DEVELOPMENT OF GASTROPODS
19
dark line takes 40 Â, the middle layer 27 Â and the inner layer 37 Â.
Inside this membrane the cytoplasm everywhere has a similar ground
structure and a similar distribution of its inclusions. In centrifuged eggs
the particles displaced by centrifugal force (lipid droplets, yolk granules,
vacuoles) are crowded until they are immediately beneath the membrane ;
no 'cortical gel layer' resisting displacement by centrifugal force can be
observed. Treatment of the eggs with LiCl in subcytolytic concentrations
does not influence the submicroscopic structure of the cytoplasm. In
higher concentrations cytolysis occurs, beginning with a disruption of
the surface membrane, and accompanied by a degradation of the
cytoplasm progressing from the surface in a centripetal direction.
Elbers concludes from his observations that lithium ions, even in
concentrations producing marked morphogenetic aberrations, do not
enter the eggs, so that they must exercise their effects by acting on or in
the surface membrane. This, together with the observations on centrifuged eggs mentioned above, makes it probable that the cortical factors
controlling morphogenesis are bound to this membrane.
VI. The Cortical Field
We have seen above (Section IV) that these cortical factors act in the
first place by directing the displacement of cytoplasmic substances
during ooplasmic segregation. Moreover, there are many indications
that the localization of nuclei and spindles is likewise dependent on the
cortex. When gastropod eggs are centrifuged some time before the
extrusion of the polar bodies, the germinal vesicle or early maturation
spindle is displaced. As a rule, it comes to lie in the zone of hyaloplasm.
In some cases, when maturation approaches the maturation spindle rises
to the surface at some point, which bears no relation to the stratification
of the egg, pushing aside the egg substances accumulated in this region.
The polar bodies are then formed at this place (e.g., Aplysia:
Raven,
1938). We may assume that the maturation spindle has returned to the
original animal pole, which has not been displaced by centrifuging. In
other cases, the polar bodies are always extruded in the hyaline zone or
immediately adjacent to it (Crepidula: Conklin, 1917;
Nudibranchia:
Costello, 1939; Physa and Limnaea: Conklin, 1910). Presumably, the
return of the maturation spindle to the original animal pole may be
hindered mechanically in these cases by the accumulated yolk substances, and the polar bodies then form at an arbitrary point of the
surface (Fig. 7). This does not mean, however, that the original polarity
of the egg has been altered, as is shown by the further development of
the eggs. At a later stage, the pronuclei or cleavage spindles return to
the original animal pole, and the first two cleavage furrows nearly
19
dark line takes 40 Â, the middle layer 27 Â and the inner layer 37 Â.
Inside this membrane the cytoplasm everywhere has a similar ground
structure and a similar distribution of its inclusions. In centrifuged eggs
the particles displaced by centrifugal force (lipid droplets, yolk granules,
vacuoles) are crowded until they are immediately beneath the membrane ;
no 'cortical gel layer' resisting displacement by centrifugal force can be
observed. Treatment of the eggs with LiCl in subcytolytic concentrations
does not influence the submicroscopic structure of the cytoplasm. In
higher concentrations cytolysis occurs, beginning with a disruption of
the surface membrane, and accompanied by a degradation of the
cytoplasm progressing from the surface in a centripetal direction.
Elbers concludes from his observations that lithium ions, even in
concentrations producing marked morphogenetic aberrations, do not
enter the eggs, so that they must exercise their effects by acting on or in
the surface membrane. This, together with the observations on centrifuged eggs mentioned above, makes it probable that the cortical factors
controlling morphogenesis are bound to this membrane.
VI. The Cortical Field
We have seen above (Section IV) that these cortical factors act in the
first place by directing the displacement of cytoplasmic substances
during ooplasmic segregation. Moreover, there are many indications
that the localization of nuclei and spindles is likewise dependent on the
cortex. When gastropod eggs are centrifuged some time before the
extrusion of the polar bodies, the germinal vesicle or early maturation
spindle is displaced. As a rule, it comes to lie in the zone of hyaloplasm.
In some cases, when maturation approaches the maturation spindle rises
to the surface at some point, which bears no relation to the stratification
of the egg, pushing aside the egg substances accumulated in this region.
The polar bodies are then formed at this place (e.g., Aplysia:
Raven,
1938). We may assume that the maturation spindle has returned to the
original animal pole, which has not been displaced by centrifuging. In
other cases, the polar bodies are always extruded in the hyaline zone or
immediately adjacent to it (Crepidula: Conklin, 1917;
Nudibranchia:
Costello, 1939; Physa and Limnaea: Conklin, 1910). Presumably, the
return of the maturation spindle to the original animal pole may be
hindered mechanically in these cases by the accumulated yolk substances, and the polar bodies then form at an arbitrary point of the
surface (Fig. 7). This does not mean, however, that the original polarity
of the egg has been altered, as is shown by the further development of
the eggs. At a later stage, the pronuclei or cleavage spindles return to
the original animal pole, and the first two cleavage furrows nearly