DEVELOPMENT OF GASTROPODS
23
this field, each type of cellular differentiation corresponding to a certain
range of values of the field factor. Lithium ions, by depressing the apex
of the gradient-field, were supposed to suppress the differentiation of the
most-animal structures, the ciliary cells of the apical plate. The cephalic
plates now originate from cells lying nearer to the animal pole, and
FIG. 8. Disturbance, by lithium, of the pattern of the head region in Limnaea stagnalis.
A. Normal trochophore, from the ventral side. B. Lithium embryo at the same stage.
a.p. apical plate, b. bridge of small cells connecting cephalic plates in lithium embryo.
c.p. cephalic plate./, foot anläge, h.v. head vesicle, m. mouth, p. prototroch (courtesy of
Ν. H. Verdonk).
eventually from a median cell group adjoining this pole. With further
intensification of the effect the cephalic plate differentiation is suppressed
in its turn, leading to complete acephaly.
This hypothesis received further support, when it appeared that a
more or less opposite effect could be obtained by a heat-shock treatment
of the eggs. The most characteristic effect of the treatment consisted in
the formation of a supernumerary cephalic plate in the dorsal mid-line of
the head, giving rise to an additional eye or a tentacle. This was explained
by the assumption that a strengthening of the cortical gradient-field,
with subsequent reduplication of its apex, had taken place (Raven et al.,
1955). The fact that CaCl 2 diminishes the effect of a heat-shock (Raven
and van Erkel, 1955) supports the view that the effect of the latter is
primarily due to its action on the cortex. Moreover, the assumption that
lithium and heat-shock affect the cortical morphogenetic field in more
or less opposite directions is corroborated by the fact that a significant
negative interaction between the two treatments with respect to the
production of head malformations could be demonstrated (Raven and
Schouten, 1962).
Recent (unpublished) observations by Ν. H. Verdonk have shown,
23
this field, each type of cellular differentiation corresponding to a certain
range of values of the field factor. Lithium ions, by depressing the apex
of the gradient-field, were supposed to suppress the differentiation of the
most-animal structures, the ciliary cells of the apical plate. The cephalic
plates now originate from cells lying nearer to the animal pole, and
FIG. 8. Disturbance, by lithium, of the pattern of the head region in Limnaea stagnalis.
A. Normal trochophore, from the ventral side. B. Lithium embryo at the same stage.
a.p. apical plate, b. bridge of small cells connecting cephalic plates in lithium embryo.
c.p. cephalic plate./, foot anläge, h.v. head vesicle, m. mouth, p. prototroch (courtesy of
Ν. H. Verdonk).
eventually from a median cell group adjoining this pole. With further
intensification of the effect the cephalic plate differentiation is suppressed
in its turn, leading to complete acephaly.
This hypothesis received further support, when it appeared that a
more or less opposite effect could be obtained by a heat-shock treatment
of the eggs. The most characteristic effect of the treatment consisted in
the formation of a supernumerary cephalic plate in the dorsal mid-line of
the head, giving rise to an additional eye or a tentacle. This was explained
by the assumption that a strengthening of the cortical gradient-field,
with subsequent reduplication of its apex, had taken place (Raven et al.,
1955). The fact that CaCl 2 diminishes the effect of a heat-shock (Raven
and van Erkel, 1955) supports the view that the effect of the latter is
primarily due to its action on the cortex. Moreover, the assumption that
lithium and heat-shock affect the cortical morphogenetic field in more
or less opposite directions is corroborated by the fact that a significant
negative interaction between the two treatments with respect to the
production of head malformations could be demonstrated (Raven and
Schouten, 1962).
Recent (unpublished) observations by Ν. H. Verdonk have shown,