28
CHR. P. RAVEN
When these exogastrulae are studied in sections, it appears that cell
differentiation has taken place to a certain extent (Raven, 1952). The
ectodermal and endodermal regions, and an intermediate 'marginal
zone' of mixed ecto-mesodermal character can be distinguished. In the
ectodermal region, large ciliated cells resembling those of the prototroch
and apical plate are often well-differentiated. In the endodermal
hemisphere, the large albumen cells of the future larval livers reach a
certain degree of differentiation. Sometimes, in the marginal zone, a
more or less atypical larval kidney has been formed. Moreover, the
stomodaeum anläge may be recognizable to a certain extent, but its
development is arrested at an early stage.
It is especially evident that the cells of larval structures exhibit a
certain degree of differentiation in the exogastrulae. However, the adult
types of tissue differentiation never occur. No ganglion cells, eyes or
statocysts, gut, radular sac, heart, muscles or shell have ever been found
in exogastrulae.
From this the conclusion may be drawn that the primary chemodifferentiation during early stages of development establishes the
general pattern of the embryo in broad outline (e.g., the difference
between ecto- and endodermal regions). Moreover, it determines the
cellular differentiation of the typical larval structures. On the other hand,
both organogenesis and histogenesis of adult organs require the intervention of new causal factors, which make their appearance during
gastrulation in consequence of the displacements of cells thereby taking
place. Circumstantial evidence that the determination of the organs of
the head in Limnaea generally takes place at the gastrula and early
trochophore stage, has been derived from a study of embryos showing
abnormal development of the foregut (Raven, 1958a).
On the mode of determination of the adult organs in gastropods very
little is known. Some experimental evidence is available with respect to
only one of them.
In normal development, the tip of the invaginating archenteron at a
certain moment makes contact with the inner side of the ectoderm in
the dorsal post-trochal region. At the point of contact, an ectodermal
thickening then forms: the shell gland. In its centre an invagination
appears, and the whole thickened area is temporarily rolled in. After
some time, it begins to evaginate again, forming an ectodermal area with
thickened margin which begins with the secretion of the shell.
In totally exogastrulated embryos of Limnaea, a shell gland is never
formed. Sometimes, however, partial exogastrulation occurs, in which
only a part of the presumptive endoderm is invaginated. In such cases a
shell gland may be formed. It is always situated in places where the wall
of the defective archenteron or a mass of invaginated endoderm cells
CHR. P. RAVEN
When these exogastrulae are studied in sections, it appears that cell
differentiation has taken place to a certain extent (Raven, 1952). The
ectodermal and endodermal regions, and an intermediate 'marginal
zone' of mixed ecto-mesodermal character can be distinguished. In the
ectodermal region, large ciliated cells resembling those of the prototroch
and apical plate are often well-differentiated. In the endodermal
hemisphere, the large albumen cells of the future larval livers reach a
certain degree of differentiation. Sometimes, in the marginal zone, a
more or less atypical larval kidney has been formed. Moreover, the
stomodaeum anläge may be recognizable to a certain extent, but its
development is arrested at an early stage.
It is especially evident that the cells of larval structures exhibit a
certain degree of differentiation in the exogastrulae. However, the adult
types of tissue differentiation never occur. No ganglion cells, eyes or
statocysts, gut, radular sac, heart, muscles or shell have ever been found
in exogastrulae.
From this the conclusion may be drawn that the primary chemodifferentiation during early stages of development establishes the
general pattern of the embryo in broad outline (e.g., the difference
between ecto- and endodermal regions). Moreover, it determines the
cellular differentiation of the typical larval structures. On the other hand,
both organogenesis and histogenesis of adult organs require the intervention of new causal factors, which make their appearance during
gastrulation in consequence of the displacements of cells thereby taking
place. Circumstantial evidence that the determination of the organs of
the head in Limnaea generally takes place at the gastrula and early
trochophore stage, has been derived from a study of embryos showing
abnormal development of the foregut (Raven, 1958a).
On the mode of determination of the adult organs in gastropods very
little is known. Some experimental evidence is available with respect to
only one of them.
In normal development, the tip of the invaginating archenteron at a
certain moment makes contact with the inner side of the ectoderm in
the dorsal post-trochal region. At the point of contact, an ectodermal
thickening then forms: the shell gland. In its centre an invagination
appears, and the whole thickened area is temporarily rolled in. After
some time, it begins to evaginate again, forming an ectodermal area with
thickened margin which begins with the secretion of the shell.
In totally exogastrulated embryos of Limnaea, a shell gland is never
formed. Sometimes, however, partial exogastrulation occurs, in which
only a part of the presumptive endoderm is invaginated. In such cases a
shell gland may be formed. It is always situated in places where the wall
of the defective archenteron or a mass of invaginated endoderm cells