SOME ASPECTS O F T H E BIOLOGY O F T H E CHAETOGNATIIS
30 I
their growth. I n the fore part there are often younger oocytes and also
oogonia. Frontal and sagittal sections of the ovary confirm the
disposition of the eggs as outlined above : anteriorly one sees the oogonia
and the smaller oocytes in different stages of the meiotic prophase;
towards the median region, the larger oocytes can be seen. The latter
may be in their last period of growth, i.e. at the end of vitellogenesis,
and clearly show the lampbrush chromosomes. Around the nucleus
there are little plates or granules which form the so-called nuclear
net, which is not constituted by chromatinic substance since it does
not stain by the Feulgen reaction but by pyroninophilous material :
we shall revert to this when dealing with the growth of oocytes
(p. 312).
Frontal sections are particularly suited for studying the inetaphase :
it is indeed not difficult to observe the typical spindle of maturation
with parallel fibres, first oriented parallel with the ovular membrane
and then perpendicular to it. On the spindle the nine bivalents can
easily be seen (Fig. 32).
Sometimes, owing to the large dimensions of the eggs, the intestine
in the region between the two ovaries has an S-bend. The ovary, as I
have said, is wrapped in a thin membrane made of flat cells whose
nuclei are also quite flat and strongly stainable. The membrane is
tightly pressed against the body wall; in the fore part it is attached t o
the lateral line by means of a thin mesentery, posteriorly it is in contact
with the seminal receptacle. The ovary is further attached t o the body
wall in the angle the latter forms with the caudal septum.
Ventral to the ovary one sees the seminal pouch, which runs as a small
canal not so long as the ovary: its anterior end is blind and is continued
posteriorly into the seminal receptacle. I n the longitudinal sections it is
possible to see the eggs inserted along the seminal pouch, by means of the
accessory fertilization cells (Fig. 12). The typical moon crescent described
in Xagitta is absent. The upper wall of the seminal pouch is only somewhat enlarged and on its epithelium the eggs are inserted. Transverse
sections show it sometimes as a circular and sometimes as a triangular
shaped lumen: this is certainly dependent on the extent t o which it
fills the pouch, and when it is strongly pressed by the mature eggs and
there are not many sperms in it, its lumen even becomes virtual.
The walls of the seminal pouch are made of the usual layers of cells
already described when dealing with Xagitta. The outer layer is a
cylindrical epithelium which is lower in the region where the seminal
pouch is connected with the ovary. The nuclei of the epithelium cells
are large and vesicular, ovoid, and with a generally evident nucleolus.
The inner layer is syncytial with scattered nuclei generally elongated
30 I
their growth. I n the fore part there are often younger oocytes and also
oogonia. Frontal and sagittal sections of the ovary confirm the
disposition of the eggs as outlined above : anteriorly one sees the oogonia
and the smaller oocytes in different stages of the meiotic prophase;
towards the median region, the larger oocytes can be seen. The latter
may be in their last period of growth, i.e. at the end of vitellogenesis,
and clearly show the lampbrush chromosomes. Around the nucleus
there are little plates or granules which form the so-called nuclear
net, which is not constituted by chromatinic substance since it does
not stain by the Feulgen reaction but by pyroninophilous material :
we shall revert to this when dealing with the growth of oocytes
(p. 312).
Frontal sections are particularly suited for studying the inetaphase :
it is indeed not difficult to observe the typical spindle of maturation
with parallel fibres, first oriented parallel with the ovular membrane
and then perpendicular to it. On the spindle the nine bivalents can
easily be seen (Fig. 32).
Sometimes, owing to the large dimensions of the eggs, the intestine
in the region between the two ovaries has an S-bend. The ovary, as I
have said, is wrapped in a thin membrane made of flat cells whose
nuclei are also quite flat and strongly stainable. The membrane is
tightly pressed against the body wall; in the fore part it is attached t o
the lateral line by means of a thin mesentery, posteriorly it is in contact
with the seminal receptacle. The ovary is further attached t o the body
wall in the angle the latter forms with the caudal septum.
Ventral to the ovary one sees the seminal pouch, which runs as a small
canal not so long as the ovary: its anterior end is blind and is continued
posteriorly into the seminal receptacle. I n the longitudinal sections it is
possible to see the eggs inserted along the seminal pouch, by means of the
accessory fertilization cells (Fig. 12). The typical moon crescent described
in Xagitta is absent. The upper wall of the seminal pouch is only somewhat enlarged and on its epithelium the eggs are inserted. Transverse
sections show it sometimes as a circular and sometimes as a triangular
shaped lumen: this is certainly dependent on the extent t o which it
fills the pouch, and when it is strongly pressed by the mature eggs and
there are not many sperms in it, its lumen even becomes virtual.
The walls of the seminal pouch are made of the usual layers of cells
already described when dealing with Xagitta. The outer layer is a
cylindrical epithelium which is lower in the region where the seminal
pouch is connected with the ovary. The nuclei of the epithelium cells
are large and vesicular, ovoid, and with a generally evident nucleolus.
The inner layer is syncytial with scattered nuclei generally elongated