8
CHR. P. RAVEN
III. The Importance of Ooplasmic Segregation
When one considers the extensive displacements and selective accumulations of cytoplasmic components during early development in gastropods, it is obvious that these processes are important for further
development. It is evident that the determination of the cells, somehow
or other, must be ultimately dependent upon the material composition
of the cytoplasm. This composition, as we have seen, becomes divergent
in different cells by a differential distribution of substances present in
the egg.
A number of experiments support the view that ooplasmic segregation
is important for cell determination.
In the first place we may mention the experiments on the morphogenetic role of the polar lobe in Ilyanassa (Crampton, 1896; Clement,
1952, 1956). As we have seen above (Section II), the substance of this
lobe is first taken up in CD, and in the next cleavage in D. In later
development the D-quadrant plays an important part ; for instance, it
produces the mesentoblast cell 4d, from which the primary mesoderm is
derived.
When the AB and CD blastomeres are isolated at the 2-cell stage, both
halves gastrulate and develop velar cilia, pigment, muscle tissue and a
ciliated enteron. But whereas CD halves may develop such structures as
shell, foot, operculum, otocysts, heart and eyes, these organs are
entirely lacking in AB halves. After isolation at the 4-cell stage, A, Β
and C quarters produce partial larvae resembling those from AB ; only D
quarters develop foot, shell and eye.
Removal of the polar lobe at the first cleavage results in larvae having
a disorganized velum and an indistinct dorsoventral polarity. There is
no primary mesoderm, but mesenchyme and muscles, probably derived
from ectomesoderm, may be present. The endoderm may show some
differentiation, but shell, foot, otocysts, eyes and heart do not develop.
From these experiments the conclusion may be drawn that the
presence of the polar lobe is necessary for normal development of the
velum, and for the formation of the primary mesoderm. In the absence
of the lobe substance, moreover, such organs as shell, foot, heart,
otocysts and eyes do not develop ; apparently the formation of these
organs is dependent, directly or indirectly, on a normal distribution of
ooplasmic substances among the cells by way of the polar lobe.
Further data on the importance of ooplasmic segregation for development results from centrifugation experiments. When eggs are centrifuged
during early cleavage, the normal distribution of substances among the
cells may be interfered with, and it may be expected that abnormal
development will be the result. This is confirmed by the experiments.
CHR. P. RAVEN
III. The Importance of Ooplasmic Segregation
When one considers the extensive displacements and selective accumulations of cytoplasmic components during early development in gastropods, it is obvious that these processes are important for further
development. It is evident that the determination of the cells, somehow
or other, must be ultimately dependent upon the material composition
of the cytoplasm. This composition, as we have seen, becomes divergent
in different cells by a differential distribution of substances present in
the egg.
A number of experiments support the view that ooplasmic segregation
is important for cell determination.
In the first place we may mention the experiments on the morphogenetic role of the polar lobe in Ilyanassa (Crampton, 1896; Clement,
1952, 1956). As we have seen above (Section II), the substance of this
lobe is first taken up in CD, and in the next cleavage in D. In later
development the D-quadrant plays an important part ; for instance, it
produces the mesentoblast cell 4d, from which the primary mesoderm is
derived.
When the AB and CD blastomeres are isolated at the 2-cell stage, both
halves gastrulate and develop velar cilia, pigment, muscle tissue and a
ciliated enteron. But whereas CD halves may develop such structures as
shell, foot, operculum, otocysts, heart and eyes, these organs are
entirely lacking in AB halves. After isolation at the 4-cell stage, A, Β
and C quarters produce partial larvae resembling those from AB ; only D
quarters develop foot, shell and eye.
Removal of the polar lobe at the first cleavage results in larvae having
a disorganized velum and an indistinct dorsoventral polarity. There is
no primary mesoderm, but mesenchyme and muscles, probably derived
from ectomesoderm, may be present. The endoderm may show some
differentiation, but shell, foot, otocysts, eyes and heart do not develop.
From these experiments the conclusion may be drawn that the
presence of the polar lobe is necessary for normal development of the
velum, and for the formation of the primary mesoderm. In the absence
of the lobe substance, moreover, such organs as shell, foot, heart,
otocysts and eyes do not develop ; apparently the formation of these
organs is dependent, directly or indirectly, on a normal distribution of
ooplasmic substances among the cells by way of the polar lobe.
Further data on the importance of ooplasmic segregation for development results from centrifugation experiments. When eggs are centrifuged
during early cleavage, the normal distribution of substances among the
cells may be interfered with, and it may be expected that abnormal
development will be the result. This is confirmed by the experiments.