MECHANISMS OF DETERMINATION IN THE DEVELOPMENT
OF GASTROPODS
CHR. P. RAVEN
Zoological Laboratory,
University of Utrecht
I. Introduction
1
II. Ooplasmic Segregation
2
III. The Importance of Ooplasmic Segregation
8
IV. The Causality of Ooplasmic Segregation
10
V. Nature, Composition and Properties of the Cortex
14
VI. The Cortical Field
19
VII. The Nature of the Cortical Morphogenese Field
22
VIII. The Origin of the Cortical Morphogenese Field
25
IX. The Causality of Cellular Differentiation
27
X. Conclusions
29
References
30
I. Introduction
The eggs of the 'Spiralia' (i.e., those groups of animals that exhibit a
spiral cleavage), according to the classical view belong to the 'mosaic
eggs', characterized by a precocious and irreversible determination of
the cells. One might expect the processes bringing about such an early
determination to differ essentially from those governing the development
in other groups, where determination occurs at a much later stage, and
the developing embryo retains for a long time the capacity of regulation.
However, investigations of the last 25 years have shown that the
differences in modes of development between the eggs of Spiralia, on the
one hand, and 'regulation eggs', like those of sea urchins and vertebrates,
on the other, are much less fundamental than was originally assumed.
At present there seems to be no doubt that the main trends of development are the same in all groups of the animal kingdom. The existing
differences can be seen as variations of the same general theme, in which
now one, then another of the component processes plays a more
prominent or more easily recognizable part. Therefore the analysis of
development in each group contributes its share to the construction of a
general picture of development. For instance, experimental embryology
of vertebrates has especially elucidated the importance of induction;
research on sea urchins has shown the role of gradients in development ;
the study of insect development made possible the analysis of gene
ι
OF GASTROPODS
CHR. P. RAVEN
Zoological Laboratory,
University of Utrecht
I. Introduction
1
II. Ooplasmic Segregation
2
III. The Importance of Ooplasmic Segregation
8
IV. The Causality of Ooplasmic Segregation
10
V. Nature, Composition and Properties of the Cortex
14
VI. The Cortical Field
19
VII. The Nature of the Cortical Morphogenese Field
22
VIII. The Origin of the Cortical Morphogenese Field
25
IX. The Causality of Cellular Differentiation
27
X. Conclusions
29
References
30
I. Introduction
The eggs of the 'Spiralia' (i.e., those groups of animals that exhibit a
spiral cleavage), according to the classical view belong to the 'mosaic
eggs', characterized by a precocious and irreversible determination of
the cells. One might expect the processes bringing about such an early
determination to differ essentially from those governing the development
in other groups, where determination occurs at a much later stage, and
the developing embryo retains for a long time the capacity of regulation.
However, investigations of the last 25 years have shown that the
differences in modes of development between the eggs of Spiralia, on the
one hand, and 'regulation eggs', like those of sea urchins and vertebrates,
on the other, are much less fundamental than was originally assumed.
At present there seems to be no doubt that the main trends of development are the same in all groups of the animal kingdom. The existing
differences can be seen as variations of the same general theme, in which
now one, then another of the component processes plays a more
prominent or more easily recognizable part. Therefore the analysis of
development in each group contributes its share to the construction of a
general picture of development. For instance, experimental embryology
of vertebrates has especially elucidated the importance of induction;
research on sea urchins has shown the role of gradients in development ;
the study of insect development made possible the analysis of gene
ι