2
C H R . P . R A V E N
actions during development. In the same way, in the study of the
development of Spiralia it is the differential distribution of cytoplasmic
substances (ooplasmic segregation) that is most striking. This does not
mean however that this process does not also occur in the other groups,
nor that the other mechanisms mentioned above are lacking in spirallycleaving eggs.
In this paper a survey of the different mechanisms of determination
in the development of the gastropods will be given (for a more extensive
treatment cf. Raven, 1958b).
II. Ooplasmic Segregation
When the fully grown oocyte in the gonad of gastropods is ready t o
ovulate, the various components and inclusions of its cytoplasm are
generally more or less evenly distributed throughout the egg (Fig. 1A).
FIG. 1. Ooplasmic segregation in Limnaea stagnalis. A. Fully grown oocyte in the
gonad. Uniform staining and distribution of cytoplasmic components. B. Fertilized egg
immediately after oviposition. First maturation spindle. Vegetal pole plasm below. Subcortical 'patches' in equatorial region at right. Staining : azan.
Distinct localizations of certain substances or particles are not observed
at this stage. Immediately after ovulation and fertilization, however, in
most cases a movement of various egg substances occurs, through which
they are accumulated or concentrated at certain places in the egg cell.
These processes, which may be summarized under the term ooplasmic
segregation, bring about a situation, in which different parts of the egg
differ more or less in their cytoplasmic composition. The ooplasmic
segregation continues during cleavage. The segregated egg substances
are distributed unequally among the cleavage cells, which in this way
C H R . P . R A V E N
actions during development. In the same way, in the study of the
development of Spiralia it is the differential distribution of cytoplasmic
substances (ooplasmic segregation) that is most striking. This does not
mean however that this process does not also occur in the other groups,
nor that the other mechanisms mentioned above are lacking in spirallycleaving eggs.
In this paper a survey of the different mechanisms of determination
in the development of the gastropods will be given (for a more extensive
treatment cf. Raven, 1958b).
II. Ooplasmic Segregation
When the fully grown oocyte in the gonad of gastropods is ready t o
ovulate, the various components and inclusions of its cytoplasm are
generally more or less evenly distributed throughout the egg (Fig. 1A).
FIG. 1. Ooplasmic segregation in Limnaea stagnalis. A. Fully grown oocyte in the
gonad. Uniform staining and distribution of cytoplasmic components. B. Fertilized egg
immediately after oviposition. First maturation spindle. Vegetal pole plasm below. Subcortical 'patches' in equatorial region at right. Staining : azan.
Distinct localizations of certain substances or particles are not observed
at this stage. Immediately after ovulation and fertilization, however, in
most cases a movement of various egg substances occurs, through which
they are accumulated or concentrated at certain places in the egg cell.
These processes, which may be summarized under the term ooplasmic
segregation, bring about a situation, in which different parts of the egg
differ more or less in their cytoplasmic composition. The ooplasmic
segregation continues during cleavage. The segregated egg substances
are distributed unequally among the cleavage cells, which in this way