DEVELOPMENT OF GASTROPODS
29
touches the inner side of the ectoderm. When the archenteron has grown
inwards in an abnormal direction, a shell gland may develop in contact
with its tip even in the pre-trochal region of the ectoderm. It may be
concluded from these observations that the formation of the shell gland
is due to an inductive action exerted by the tip of the archenteron on the
ectoderm with which it makes contact (Raven, 1952). Observations by
Hess (1957) seem to show that even a few endoderm cells suffice for
induction of the shell gland.
Similar relationships have been found in Bithynia (Hess, 1956a, b).
In exogastrulae of this species a belated invagination of part of the
endoderm, probably corresponding to the larval liver of normal embryos, occurs. In those cases a shell gland is formed at the place of
contact between the invaginated endoderm and the ectoderm. Whilst it
has already appeared after 3 days in normal embryos, in exogastrulae it
is formed only after 6 or 7 days, when the endodermal invagination has
taken place. The fact that after isolation of blastomeres at the 2-cell
stage both AB- and CD-embryos are able to form a shell gland, forms
corroborating evidence that shell-gland formation is not due to selfdifferentiation from a predetermined area, but to contact induction.
Though we may conjecture that induction phenomena play an important part in the determination of other adult organs in gastropods,
nothing more definite can be said at the moment.
X. Conclusions
The following picture of early development in gastropods may be
drawn. Starting from the roughly uniform distribution of the cytoplasmic
constituents in the fully grown oocyte, local accumulations of cytoplasmic substances take place by interactions with the cortex. The
latter carries a morphogenetic field, based on local differences in its
structure and properties, which has in part a mosaic character. It has
been established during oogenesis by the interaction of the oocyte with
the surrounding elements of the gonad. The cortical field not only
controls the ooplasmic segregation, but also determines the direction and
position of cleavage spindles, and thereby the pattern of cleavage. The
cells are thus equipped from the outset with a different chemical
composition of their cytoplasm, and moreover with different portions of
the original cortical mosaic. This, in conjunction with the nuclear genes
now coming into operation, determines their further development. This
does not mean that the differentiation of all cells is irrevocably fixed by
this early determinative mechanism. Only the general body plan of the
embryo in broad outline is laid down, apparently by differences in
competence among the cells in various regions. Moreover, in some areas
B2
29
touches the inner side of the ectoderm. When the archenteron has grown
inwards in an abnormal direction, a shell gland may develop in contact
with its tip even in the pre-trochal region of the ectoderm. It may be
concluded from these observations that the formation of the shell gland
is due to an inductive action exerted by the tip of the archenteron on the
ectoderm with which it makes contact (Raven, 1952). Observations by
Hess (1957) seem to show that even a few endoderm cells suffice for
induction of the shell gland.
Similar relationships have been found in Bithynia (Hess, 1956a, b).
In exogastrulae of this species a belated invagination of part of the
endoderm, probably corresponding to the larval liver of normal embryos, occurs. In those cases a shell gland is formed at the place of
contact between the invaginated endoderm and the ectoderm. Whilst it
has already appeared after 3 days in normal embryos, in exogastrulae it
is formed only after 6 or 7 days, when the endodermal invagination has
taken place. The fact that after isolation of blastomeres at the 2-cell
stage both AB- and CD-embryos are able to form a shell gland, forms
corroborating evidence that shell-gland formation is not due to selfdifferentiation from a predetermined area, but to contact induction.
Though we may conjecture that induction phenomena play an important part in the determination of other adult organs in gastropods,
nothing more definite can be said at the moment.
X. Conclusions
The following picture of early development in gastropods may be
drawn. Starting from the roughly uniform distribution of the cytoplasmic
constituents in the fully grown oocyte, local accumulations of cytoplasmic substances take place by interactions with the cortex. The
latter carries a morphogenetic field, based on local differences in its
structure and properties, which has in part a mosaic character. It has
been established during oogenesis by the interaction of the oocyte with
the surrounding elements of the gonad. The cortical field not only
controls the ooplasmic segregation, but also determines the direction and
position of cleavage spindles, and thereby the pattern of cleavage. The
cells are thus equipped from the outset with a different chemical
composition of their cytoplasm, and moreover with different portions of
the original cortical mosaic. This, in conjunction with the nuclear genes
now coming into operation, determines their further development. This
does not mean that the differentiation of all cells is irrevocably fixed by
this early determinative mechanism. Only the general body plan of the
embryo in broad outline is laid down, apparently by differences in
competence among the cells in various regions. Moreover, in some areas
B2