30
CHR. P. RAVEN
this primary chemo-differentiation also suffices for the cellular differentiation of larval structures. Finally, it conditions the local changes in
cell shape and cell affinity, which bring about the morphogenetic
movements leading to gastrulation and formation of the germ layers.
But the organogenesis and histogenesis of the adult organs require the
intervention of new causal factors, which only appear during gastrulation consequent on the attainment of new topographical relationships
between the cells. As regards the nature of the interactions thereby
established, our knowledge is still very restricted. The data on shellgland development seem to show, however, that processes of contact
induction may play a part.
Two remarks can be made in this connection. In the first place, little
has remained of the classical picture of the molluscan egg as a 'mosaic'
egg. One can hardly speak of a precocious and irreversible celldetermination at early cleavage stages in gastropods. The spatial
multiplicity of the adult animal is only gradually built up during
ontogenesis by a dynamic system of directed movements.
This also means that the difference in mode of development between a
gastropod egg and a 'regulation egg' like those of the Amphibia, is much
less pronounced than was originally assumed. In both groups, the
determination of the adult organs takes place during and shortly after
gastrulation. Development prior to gastrulation mainly serves to plot
out the main germ regions with their different competences and dynamic
(topogenetic) properties. This fundamental resemblance has long been
obscured by the fact that some cellular differentiations, characteristic of
typical larval organs, functioning at an early stage for the locomotion,
nutrition and excretion of the larva, show a precocious determination in
the Spiralia. However, analogous relationships are not entirely lacking
in vertebrates (cf. the differentiation of adhesive gland epithelium from
explanted early gastrula ectoderm in anurans). The main difference
between gastropods, on the one hand, and Amphibia, on the other, thus
seems to be that research on mechanisms of determination in gastropods
up till now has chiefly been concerned with the early phase preceding
gastrulation, whereas the main attention in Amphibia has been directed
towards later phases of the determination process. This difference in
emphasis does not however indicate very fundamental differences in
mechanisms of determination.
References
Attardo, C. (1955a). Ric. sei. 25, 2797.
Attardo, C. (1955b). R. C. Accad. Lincei, CI. Sei. (8) 19, 83.
Attardo, C. (1957). Acta Embryol. Morphol. exp. 1, 65.
Blochmann, F. (1882). Z. wise. Zool. 36, 125.
CHR. P. RAVEN
this primary chemo-differentiation also suffices for the cellular differentiation of larval structures. Finally, it conditions the local changes in
cell shape and cell affinity, which bring about the morphogenetic
movements leading to gastrulation and formation of the germ layers.
But the organogenesis and histogenesis of the adult organs require the
intervention of new causal factors, which only appear during gastrulation consequent on the attainment of new topographical relationships
between the cells. As regards the nature of the interactions thereby
established, our knowledge is still very restricted. The data on shellgland development seem to show, however, that processes of contact
induction may play a part.
Two remarks can be made in this connection. In the first place, little
has remained of the classical picture of the molluscan egg as a 'mosaic'
egg. One can hardly speak of a precocious and irreversible celldetermination at early cleavage stages in gastropods. The spatial
multiplicity of the adult animal is only gradually built up during
ontogenesis by a dynamic system of directed movements.
This also means that the difference in mode of development between a
gastropod egg and a 'regulation egg' like those of the Amphibia, is much
less pronounced than was originally assumed. In both groups, the
determination of the adult organs takes place during and shortly after
gastrulation. Development prior to gastrulation mainly serves to plot
out the main germ regions with their different competences and dynamic
(topogenetic) properties. This fundamental resemblance has long been
obscured by the fact that some cellular differentiations, characteristic of
typical larval organs, functioning at an early stage for the locomotion,
nutrition and excretion of the larva, show a precocious determination in
the Spiralia. However, analogous relationships are not entirely lacking
in vertebrates (cf. the differentiation of adhesive gland epithelium from
explanted early gastrula ectoderm in anurans). The main difference
between gastropods, on the one hand, and Amphibia, on the other, thus
seems to be that research on mechanisms of determination in gastropods
up till now has chiefly been concerned with the early phase preceding
gastrulation, whereas the main attention in Amphibia has been directed
towards later phases of the determination process. This difference in
emphasis does not however indicate very fundamental differences in
mechanisms of determination.
References
Attardo, C. (1955a). Ric. sei. 25, 2797.
Attardo, C. (1955b). R. C. Accad. Lincei, CI. Sei. (8) 19, 83.
Attardo, C. (1957). Acta Embryol. Morphol. exp. 1, 65.
Blochmann, F. (1882). Z. wise. Zool. 36, 125.