SOME ASPECTS O F T H E BIOLOGY OF THE CHAETOGNATHS
335
E. Laying of the eggs
1. Laying in Sagitta
As we have said (p. 299), fertilization of the eggs is brought about
by spermatozoa which go through the accessory fertilization cells. The
eggs, after having been fertilized, pass through the syncytial wall of the
seminal pouch and insinuate themselves between this syncytial wall
and the epithelial wall, thus forming the so-called temporary oviduct
(Stevens, 1905, 1910 ; Bord&s, 1920 ; Ghirardelli, 1959d). The laying of
the egg differs somewhat in the various species that have bccn studied ;
but in any case, as far as we know, they always emerge into the open
by making their way through the tissues of the pouch or seminal
receptacle.
I have had more than once the opportunity of observing the laying
of the eggs both in Sagitta inJlata and 8. bipune~ata at the Zoological
Stations at Naples and at Villefranche-sur-Mer. When the eggs are on
the point of being laid, the nuclear membrane disappears. The eggs are
a t this moment in the first metaphase and they become more opaque
than usual. About half an hour after the membrane has disappeared,
the egg begins passing through the temporary oviduct. It is not always
easy to follow this movement when several eggs are concerned: they
move simultaneously and the appearance of the ovary is being much
modified. Having entered the temporary oviduct the eggs are compressed against each other, assuming somewhat polyhedral contours
(Fig. 27). Usually when the eggs start their descent along the oviduct
they continue to change shape, often with the regular rhythm first
observed by Stevens (1910). It is, nevertheless, possible that the eggs
stay in the oviduct for a certain amount of time before laying, and
that in unhealthy specimens they remain there for several hours
(perhaps three, four or moro). I n some instances even several eggs can
be seen descending along the duct, in others only one egg is observed
and this is noticeably elongated so as to occupy a space which might
bc as long as one-half of the ovary. I n some cases, the presence of
bubbles (Stevens, 1910) can be noted at the poles of the egg. Stevens
bclieved them to be air, but they are instead of a moderately viscose
substance. When the forward bubble reaches the genital papilla, the
first muscular movements of the body of the animal occur : the genital
pore seems to open suddenly, the bubble comes out and breaks,
immediately followed by the egg.
According to Stevens, certain times are preferred for the laying of
eggs in the various species of chaetognaths. Thus, while 8. elegans
preferabIy Iays between 11.00 and 18.00 h, 8. bipunetata lays at sunset;
335
E. Laying of the eggs
1. Laying in Sagitta
As we have said (p. 299), fertilization of the eggs is brought about
by spermatozoa which go through the accessory fertilization cells. The
eggs, after having been fertilized, pass through the syncytial wall of the
seminal pouch and insinuate themselves between this syncytial wall
and the epithelial wall, thus forming the so-called temporary oviduct
(Stevens, 1905, 1910 ; Bord&s, 1920 ; Ghirardelli, 1959d). The laying of
the egg differs somewhat in the various species that have bccn studied ;
but in any case, as far as we know, they always emerge into the open
by making their way through the tissues of the pouch or seminal
receptacle.
I have had more than once the opportunity of observing the laying
of the eggs both in Sagitta inJlata and 8. bipune~ata at the Zoological
Stations at Naples and at Villefranche-sur-Mer. When the eggs are on
the point of being laid, the nuclear membrane disappears. The eggs are
a t this moment in the first metaphase and they become more opaque
than usual. About half an hour after the membrane has disappeared,
the egg begins passing through the temporary oviduct. It is not always
easy to follow this movement when several eggs are concerned: they
move simultaneously and the appearance of the ovary is being much
modified. Having entered the temporary oviduct the eggs are compressed against each other, assuming somewhat polyhedral contours
(Fig. 27). Usually when the eggs start their descent along the oviduct
they continue to change shape, often with the regular rhythm first
observed by Stevens (1910). It is, nevertheless, possible that the eggs
stay in the oviduct for a certain amount of time before laying, and
that in unhealthy specimens they remain there for several hours
(perhaps three, four or moro). I n some instances even several eggs can
be seen descending along the duct, in others only one egg is observed
and this is noticeably elongated so as to occupy a space which might
bc as long as one-half of the ovary. I n some cases, the presence of
bubbles (Stevens, 1910) can be noted at the poles of the egg. Stevens
bclieved them to be air, but they are instead of a moderately viscose
substance. When the forward bubble reaches the genital papilla, the
first muscular movements of the body of the animal occur : the genital
pore seems to open suddenly, the bubble comes out and breaks,
immediately followed by the egg.
According to Stevens, certain times are preferred for the laying of
eggs in the various species of chaetognaths. Thus, while 8. elegans
preferabIy Iays between 11.00 and 18.00 h, 8. bipunetata lays at sunset;