340
ELVEZIO GHIRARDELLI
By using certain special devices it has, nevertheless, been possible
to observe the phenomenon more than once (Ghirardelli, 1956a, 1959d).
The observations have been effected by placing Spadella in good condition in micro-aquaria which were prepared by using cavity slides
whose depression was filled with filtered sea water and closed with a
coverslip. By taking care that an air bubble remained under the cover,
one limited the space at the disposal of the Spadella, and at the same
time a sufficient supply of oxygen was provided for the duration of the
experiment (Ghirardelli, 1956a). It has thus been possible t o use
enlargements high enough ( x 400-900) to allow us to follow easily the
laying of the eggs under the phase contrast microscope. I n Spadella,
contrary t o what occurs with Sagitta, it is not always possible to
foresee the moment of laying ; the disappearance of the nuclear membrane is not a sign of the same significance in Spadella as in Xagitta.
While in Sagitta the disappearance of the nuclear membrane normally
precedes the laying of the eggs by some 10 min or 1 h at the most,
several hours may pass before the laying begins in Spadella. The eggs
of Xpadella when nearing the time of spawning can be recognized by
their size (about 250 p diameter), their noticeable opacity due to the
cytoplasm being uniformly full of vitelline granules, and also by the
disappearance of the nuclear membrane. I n each ovary the number of
mature eggs varies from one to a dozen, in relation t o the season and
the stage of maturity of the specimen. The greatest number of eggs
can be seen in spring (Ghirardelli, 1959c, 1963a). Also in Spadella as in
Sugitta, the eggs before being laid appear compressed by one another
and may take a polyhedral shape. Consequent on movements of the
intestine, due also to the ingestion of food and water, the eggs change
their shape t o a remarkable extent, showing that they are extremely
elastic. The temperature a t which spawning occurs is between 18" and
21°C. Higher temperatures, around 25-26"C7 tend rather to inhibit
than to hasten the phenomenon.The laying of the eggs, as far as I
could see, occurs in the following manner.
With nothing to give a hint of the imminence of the occurrence, all
the eggs contained in the ovary suddenly start moving towards its
posterior end. The same phenomenon can be seen simultaneously in
both ovaries. As soon as the first egg touches the outer wall of the
so-called vagina, a small protuberance rises from the latter, the centre
of which is afterwards depressed, thus forming a small cup into which
the egg settles, its anterior end lying in the direction of its movement.
I n the meantime the egg has become pear-shaped and its more pointed
end slips between the epithelial cells of the mall of the vagina which
are pushed aside in all directions, so that the egg seems t o penetrate
ELVEZIO GHIRARDELLI
By using certain special devices it has, nevertheless, been possible
to observe the phenomenon more than once (Ghirardelli, 1956a, 1959d).
The observations have been effected by placing Spadella in good condition in micro-aquaria which were prepared by using cavity slides
whose depression was filled with filtered sea water and closed with a
coverslip. By taking care that an air bubble remained under the cover,
one limited the space at the disposal of the Spadella, and at the same
time a sufficient supply of oxygen was provided for the duration of the
experiment (Ghirardelli, 1956a). It has thus been possible t o use
enlargements high enough ( x 400-900) to allow us to follow easily the
laying of the eggs under the phase contrast microscope. I n Spadella,
contrary t o what occurs with Sagitta, it is not always possible to
foresee the moment of laying ; the disappearance of the nuclear membrane is not a sign of the same significance in Spadella as in Xagitta.
While in Sagitta the disappearance of the nuclear membrane normally
precedes the laying of the eggs by some 10 min or 1 h at the most,
several hours may pass before the laying begins in Spadella. The eggs
of Xpadella when nearing the time of spawning can be recognized by
their size (about 250 p diameter), their noticeable opacity due to the
cytoplasm being uniformly full of vitelline granules, and also by the
disappearance of the nuclear membrane. I n each ovary the number of
mature eggs varies from one to a dozen, in relation t o the season and
the stage of maturity of the specimen. The greatest number of eggs
can be seen in spring (Ghirardelli, 1959c, 1963a). Also in Spadella as in
Sugitta, the eggs before being laid appear compressed by one another
and may take a polyhedral shape. Consequent on movements of the
intestine, due also to the ingestion of food and water, the eggs change
their shape t o a remarkable extent, showing that they are extremely
elastic. The temperature a t which spawning occurs is between 18" and
21°C. Higher temperatures, around 25-26"C7 tend rather to inhibit
than to hasten the phenomenon.The laying of the eggs, as far as I
could see, occurs in the following manner.
With nothing to give a hint of the imminence of the occurrence, all
the eggs contained in the ovary suddenly start moving towards its
posterior end. The same phenomenon can be seen simultaneously in
both ovaries. As soon as the first egg touches the outer wall of the
so-called vagina, a small protuberance rises from the latter, the centre
of which is afterwards depressed, thus forming a small cup into which
the egg settles, its anterior end lying in the direction of its movement.
I n the meantime the egg has become pear-shaped and its more pointed
end slips between the epithelial cells of the mall of the vagina which
are pushed aside in all directions, so that the egg seems t o penetrate