SOME ASPECTS O F THE BIOLOGY O F T H E CHAETOGNATHS
305
deeply penetrating the thickness of the wall, often ends by jutting into
the lumen of the seminal pouch
As development proceeds. the accessory fertilization cells vacuolize
and quite probably these vacuoli, commuiiicating with each other,
allow the passage of the spermatozoon. When the vacuoli open into the
seminal pouch, a plug of coagulable substance forms in correspondence
to the insertion of the fertilization apparatus, which is in all respects
similar to that observed in Sagitta. This apparatus allows the transit
of the spermatozoon from the seminal pouch to the egg. This passage
must be very rapid: only on a few occasions have I been able to see
the spermatozoon engaged in the vacuoli which pass through the cells.
Also in Spadella the fate of the fertilization apparatus after the
passage of the spermiuin remains obscure. m'hile in some cases it seems
that one cell at least remains in the egg, in other more numerous cases
one can be sure that the biggest oocytes lose any contact with the
fecundation cell immediately after the penetration of the spermatozoon.
It is confirmed that the fertilization apparatus, contrary to what
Vasiljev believed, does not originate from elements of oogonial nature
but derives from the epithelium of the pouch.
4. The female reproductive apparatus in Pterosagitta
The structure of the female genital apparatus in Pterosagitta
(Ghirardelli, 1953e) closely resembles that observed in Sagitta and
Xpadella. I n young specimens-in which the eggs have not yet reached
full development or the scminal pouches are not completely full-and
in the caudal portion of the ovary of mature specimens, a crescent like
the one described in Sagitta can be observed. The wings of the crescent
are, however, shorter and the cells are irregularly disposed. Around
the eggs there is always an abundant secretion of hyaline substance.
At the centre of the crescent there is the small canal formed, as usual,
by two layers of cells, the outer one epithelial, the one more to the
inside syncytial.
The ampullae at the anterior end of each seminal pouch are the
most interesting features in the female genital apparatus of Pterosagitta :
when they are full of sperms they look in transmitted light like two
voluminous ovoidal masses, about 1 mm long, placed on the level of
the anterior extremities of the ovary. They are homologous with the
anterior end of the seminal pouch in Sagitta and Xpadella, and very
similar to the anterior region of the seminal pouch of Xagitta decipiens.
They are not homologous with the receptacles of Spadella which
correspond to the posterior region of the pouch. The two ampullae of
Pterosagitta are always far from the caudal septum, but owing to their
305
deeply penetrating the thickness of the wall, often ends by jutting into
the lumen of the seminal pouch
As development proceeds. the accessory fertilization cells vacuolize
and quite probably these vacuoli, commuiiicating with each other,
allow the passage of the spermatozoon. When the vacuoli open into the
seminal pouch, a plug of coagulable substance forms in correspondence
to the insertion of the fertilization apparatus, which is in all respects
similar to that observed in Sagitta. This apparatus allows the transit
of the spermatozoon from the seminal pouch to the egg. This passage
must be very rapid: only on a few occasions have I been able to see
the spermatozoon engaged in the vacuoli which pass through the cells.
Also in Spadella the fate of the fertilization apparatus after the
passage of the spermiuin remains obscure. m'hile in some cases it seems
that one cell at least remains in the egg, in other more numerous cases
one can be sure that the biggest oocytes lose any contact with the
fecundation cell immediately after the penetration of the spermatozoon.
It is confirmed that the fertilization apparatus, contrary to what
Vasiljev believed, does not originate from elements of oogonial nature
but derives from the epithelium of the pouch.
4. The female reproductive apparatus in Pterosagitta
The structure of the female genital apparatus in Pterosagitta
(Ghirardelli, 1953e) closely resembles that observed in Sagitta and
Xpadella. I n young specimens-in which the eggs have not yet reached
full development or the scminal pouches are not completely full-and
in the caudal portion of the ovary of mature specimens, a crescent like
the one described in Sagitta can be observed. The wings of the crescent
are, however, shorter and the cells are irregularly disposed. Around
the eggs there is always an abundant secretion of hyaline substance.
At the centre of the crescent there is the small canal formed, as usual,
by two layers of cells, the outer one epithelial, the one more to the
inside syncytial.
The ampullae at the anterior end of each seminal pouch are the
most interesting features in the female genital apparatus of Pterosagitta :
when they are full of sperms they look in transmitted light like two
voluminous ovoidal masses, about 1 mm long, placed on the level of
the anterior extremities of the ovary. They are homologous with the
anterior end of the seminal pouch in Sagitta and Xpadella, and very
similar to the anterior region of the seminal pouch of Xagitta decipiens.
They are not homologous with the receptacles of Spadella which
correspond to the posterior region of the pouch. The two ampullae of
Pterosagitta are always far from the caudal septum, but owing to their