26
CHR. P. RAVEN
sinistral races, differing in respect of one Mendelian allelomorph. The
direction of coiling of an individual is not dependent on its own genotype,
however, but on that of its female parent (Sturtevant, 1923). The
asymmetry of the egg structure, which is responsible for the direction of
cleavage and, thereby, for that of coiling, is apparently laid down in the
immature egg during oogenesis. Since we know that the direction of
cleavage spindles is generally dependent on cortical factors, it seems
probable that this asymmetry is due to an asymmetrical or spiral
structure of the egg cortex.
The cortical morphogenetic field might be built up during oogenesis
by an autonomous patterning process inside the oocyte under the
control of the oocyte nucleus. On the other hand, structures outside the
oocyte might take part in the process.
There are many indications that in molluscs generally the polarity of
the egg is related to its position in the ovary. The place of attachment
of the growing oocyte to the wall of the gonad becomes the vegetal side of
the egg. This is especially clear in those cases, where there remains after
ovulation a scar of the egg stalk in the primary egg membrane, the socalled micropyle; this is (e.g., in Lamellibranchiata and many Gastropoda) always situated at the vegetal pole.
These observations could be explained by the assumption that the
polarity of the egg cell (and thus the main direction of its cortical field)
is determined epigenetically by its surroundings, e.g., by the direction
in which the food stream reaches the growing oocyte. On the other hand,
however, it might be argued that the position of the oocyte in the gonad
is determined by its inherent polarity as a derivative of a germinal
epithelium cell.
However, our recent observations on Limnaea
stagnalis
clearly
indicate that the details of the cortical pattern of the fertilized egg
reflect peculiarities in the mutual positions of elements surrounding the
oocyte in the gonad. The oocytes lie with one side flat against the wall
of the gonad acinus, whereas the other side protrudes into the lumen
and is surrounded by a follicle. The inner layer of the follicle consists,
in older oocytes, of a whorl of six cells. They are arranged in a very
characteristic way, three lying near together, a fourth at a small
distance in an anti-clockwise direction, whereas the other two cells are
separated by greater distances (Raven, 1963) (Fig. 10). The oocyte
together with its surrounding structures forms a pattern that is at the
same time polar, dorsoventral, and asymmetric. This pattern shows a
striking similarity to the one revealed by the cytoplasmic differentiations
of the recently laid egg (Fig. 9), and it is obvious to assume that one is
the consequence of the other. The vegetal pole plasm of the recently laid
egg corresponds in its extension with that part of the egg cortex which
CHR. P. RAVEN
sinistral races, differing in respect of one Mendelian allelomorph. The
direction of coiling of an individual is not dependent on its own genotype,
however, but on that of its female parent (Sturtevant, 1923). The
asymmetry of the egg structure, which is responsible for the direction of
cleavage and, thereby, for that of coiling, is apparently laid down in the
immature egg during oogenesis. Since we know that the direction of
cleavage spindles is generally dependent on cortical factors, it seems
probable that this asymmetry is due to an asymmetrical or spiral
structure of the egg cortex.
The cortical morphogenetic field might be built up during oogenesis
by an autonomous patterning process inside the oocyte under the
control of the oocyte nucleus. On the other hand, structures outside the
oocyte might take part in the process.
There are many indications that in molluscs generally the polarity of
the egg is related to its position in the ovary. The place of attachment
of the growing oocyte to the wall of the gonad becomes the vegetal side of
the egg. This is especially clear in those cases, where there remains after
ovulation a scar of the egg stalk in the primary egg membrane, the socalled micropyle; this is (e.g., in Lamellibranchiata and many Gastropoda) always situated at the vegetal pole.
These observations could be explained by the assumption that the
polarity of the egg cell (and thus the main direction of its cortical field)
is determined epigenetically by its surroundings, e.g., by the direction
in which the food stream reaches the growing oocyte. On the other hand,
however, it might be argued that the position of the oocyte in the gonad
is determined by its inherent polarity as a derivative of a germinal
epithelium cell.
However, our recent observations on Limnaea
stagnalis
clearly
indicate that the details of the cortical pattern of the fertilized egg
reflect peculiarities in the mutual positions of elements surrounding the
oocyte in the gonad. The oocytes lie with one side flat against the wall
of the gonad acinus, whereas the other side protrudes into the lumen
and is surrounded by a follicle. The inner layer of the follicle consists,
in older oocytes, of a whorl of six cells. They are arranged in a very
characteristic way, three lying near together, a fourth at a small
distance in an anti-clockwise direction, whereas the other two cells are
separated by greater distances (Raven, 1963) (Fig. 10). The oocyte
together with its surrounding structures forms a pattern that is at the
same time polar, dorsoventral, and asymmetric. This pattern shows a
striking similarity to the one revealed by the cytoplasmic differentiations
of the recently laid egg (Fig. 9), and it is obvious to assume that one is
the consequence of the other. The vegetal pole plasm of the recently laid
egg corresponds in its extension with that part of the egg cortex which