328
ELVEZIO GHIRARDELLI
experimentally.
However, self-fertilization seems difficult under
natural circumstances, since it cannot be seen how the spermatozoa
produced by a Spadella can reach the orifices of the seminal receptacles
of thc same individual, considering the tendency they normally have
to niigrate-when they are on the caudal region of the body-towards
the posterior fin rather than towards the orifices of the receptacula
seminis.
As far as the observations of other workers are concerned, though
one cannot discard the possibility that the behaviour of the Spadella
from Plymouth (John, 1933) and from Helsingm may differ somewhat;
it cannot be excluded that in view of the speed with which the facts
we described occur, some phases of the above phenomena may have
escaped the observers.
The factors which influence the migration-which as we have seen
seem to follow a pre-arranged course-remain to be ascertained. The
integrity of the teguments is an essential condition but on the dorsal
region of Spadella there are no visible structures-at least not visible
through the normal means of observation-to which one may with
certainty attribute a determining function in leading the sperms
towards the orifice of the seminal receptacles. The ciliary loop certainly
plays an important role in regulating the migration of the spermatozoa :
actually, depending on the position in which the spermatophore is laid
in respect to the corona ciliata, the migration which follows is regular
or is not. Beheading with removal of the corona does not as a rule
stop the migration, provided the spermatophore is placed on the neck
immediately after the cutting has been done.
I n connexion with the probable influence of the corona, it has been
supposed that the migration of the sperms may be regulated by a
chemotactic action, by some substance produced by the corona, which
continually spreads along the back of the animal. This could explain
how the regular migration of the sperms may also be possible in animals
which are not mating, in other words at any time when a spermatophore
is placed on the neck behind the ciliary loop. It could also explain why
the migration ceases some time after the specimen has been beheaded,
i.e. deprived of the corona and of the secretion it produces. It seems
therefore logical at this point to relate the peculiarities of the migration
of the spermatozoa on the back of Spadella to the presence of the
secretion produced by the corona which spreads on to the back,
massing in noteworthy quantity near the genital orifices (see p. 284).
This observation is confirmed by Di Marcotullio (1966).
This could also explain the greater activity shown by spermatozoa
when near the genital orifices and the change of direction of the
ELVEZIO GHIRARDELLI
experimentally.
However, self-fertilization seems difficult under
natural circumstances, since it cannot be seen how the spermatozoa
produced by a Spadella can reach the orifices of the seminal receptacles
of thc same individual, considering the tendency they normally have
to niigrate-when they are on the caudal region of the body-towards
the posterior fin rather than towards the orifices of the receptacula
seminis.
As far as the observations of other workers are concerned, though
one cannot discard the possibility that the behaviour of the Spadella
from Plymouth (John, 1933) and from Helsingm may differ somewhat;
it cannot be excluded that in view of the speed with which the facts
we described occur, some phases of the above phenomena may have
escaped the observers.
The factors which influence the migration-which as we have seen
seem to follow a pre-arranged course-remain to be ascertained. The
integrity of the teguments is an essential condition but on the dorsal
region of Spadella there are no visible structures-at least not visible
through the normal means of observation-to which one may with
certainty attribute a determining function in leading the sperms
towards the orifice of the seminal receptacles. The ciliary loop certainly
plays an important role in regulating the migration of the spermatozoa :
actually, depending on the position in which the spermatophore is laid
in respect to the corona ciliata, the migration which follows is regular
or is not. Beheading with removal of the corona does not as a rule
stop the migration, provided the spermatophore is placed on the neck
immediately after the cutting has been done.
I n connexion with the probable influence of the corona, it has been
supposed that the migration of the sperms may be regulated by a
chemotactic action, by some substance produced by the corona, which
continually spreads along the back of the animal. This could explain
how the regular migration of the sperms may also be possible in animals
which are not mating, in other words at any time when a spermatophore
is placed on the neck behind the ciliary loop. It could also explain why
the migration ceases some time after the specimen has been beheaded,
i.e. deprived of the corona and of the secretion it produces. It seems
therefore logical at this point to relate the peculiarities of the migration
of the spermatozoa on the back of Spadella to the presence of the
secretion produced by the corona which spreads on to the back,
massing in noteworthy quantity near the genital orifices (see p. 284).
This observation is confirmed by Di Marcotullio (1966).
This could also explain the greater activity shown by spermatozoa
when near the genital orifices and the change of direction of the