SOME ASPECTS OF THE BIOLOGY O F THE CHAETOGNATHS
311
centriole ; there is an exterior chromatic thread, normally placed
laterally to the head of the sperm (which is also quite elongated and
thin) connected to the anterior centriole. After acetic fixation, the
chromatic thread is no longer applied against the head, and it can have
the aspect of an undulating membrane. The middle segment is short
by comparison with the length of the head and so is the caudal
flagellum.
A different description of spermatozoa in chaetognaths was given
by Retzius (1909) : after maceration, he observed that around the
spermatozoon there were two or even three thin threads. He further
wrongly considered as anterior the side which has been later described
as posterior. Retzius’ researches have now only a historical value.
Jagersten ( 1940) also has remarked that spermatozoa when isolated
in water are practically unable to move ; on the contrary, when they are
massed together they show lively movements and the masses of sperms
can give rise to prolongations in different directions, whose methods of
formation have been accurately described by Jagersten. The movements of the masses of spermatozoa undoubtedly have a remarkable
importance in fertilization, as we shall see later.
2. Oogenesis and growth of the oocytes
Maturation and growth of the oocytes have been described especially
by Stevens (1903) and BordAs (1920) who showed that in an ovary there
might be oocytes of various dimensions. A structural peculiarity shown
by the above authors is the so-called nuclear net. It is a series of grains
and small plaques which can be seen in oocytes in their first period of
growth, with a diameter of about 10-15 p, whose nuclei have reached
the late pachytene. I n correspondence to the nuclear membrane, one
observes some peculiar granulations strongly stainable with ferric
haematoxylin and pyroninophilous : they are completely Feulgen
negative. These structures seem t o be abundantly supplied with
ribonucleoproteins, few polysaccharides and a protein stroma which
determines the persistence of stainability with iron haematoxylin and
also after hydrolysis with 10% perchloric acid for 24 h at 0°C. Contrary
to former belief there is no nucleolus in the eggs of Spadella cephaloptera
(Ghirardelli and Brandi, 1961 ; Ghirardelli, 1961b). It is very probable
that the functions of the nucleolus are taken by the small plaques of
the so-called nuclear net.
At the moment that these granulations begin to appear they are
certainly on the inside of the nuclear membrane, but later they seem
to be within the membrane itself, sometimes giving the impression that
they enter into direct contact with the cytoplasm. When they have
A.M.B.-6
11
311
centriole ; there is an exterior chromatic thread, normally placed
laterally to the head of the sperm (which is also quite elongated and
thin) connected to the anterior centriole. After acetic fixation, the
chromatic thread is no longer applied against the head, and it can have
the aspect of an undulating membrane. The middle segment is short
by comparison with the length of the head and so is the caudal
flagellum.
A different description of spermatozoa in chaetognaths was given
by Retzius (1909) : after maceration, he observed that around the
spermatozoon there were two or even three thin threads. He further
wrongly considered as anterior the side which has been later described
as posterior. Retzius’ researches have now only a historical value.
Jagersten ( 1940) also has remarked that spermatozoa when isolated
in water are practically unable to move ; on the contrary, when they are
massed together they show lively movements and the masses of sperms
can give rise to prolongations in different directions, whose methods of
formation have been accurately described by Jagersten. The movements of the masses of spermatozoa undoubtedly have a remarkable
importance in fertilization, as we shall see later.
2. Oogenesis and growth of the oocytes
Maturation and growth of the oocytes have been described especially
by Stevens (1903) and BordAs (1920) who showed that in an ovary there
might be oocytes of various dimensions. A structural peculiarity shown
by the above authors is the so-called nuclear net. It is a series of grains
and small plaques which can be seen in oocytes in their first period of
growth, with a diameter of about 10-15 p, whose nuclei have reached
the late pachytene. I n correspondence to the nuclear membrane, one
observes some peculiar granulations strongly stainable with ferric
haematoxylin and pyroninophilous : they are completely Feulgen
negative. These structures seem t o be abundantly supplied with
ribonucleoproteins, few polysaccharides and a protein stroma which
determines the persistence of stainability with iron haematoxylin and
also after hydrolysis with 10% perchloric acid for 24 h at 0°C. Contrary
to former belief there is no nucleolus in the eggs of Spadella cephaloptera
(Ghirardelli and Brandi, 1961 ; Ghirardelli, 1961b). It is very probable
that the functions of the nucleolus are taken by the small plaques of
the so-called nuclear net.
At the moment that these granulations begin to appear they are
certainly on the inside of the nuclear membrane, but later they seem
to be within the membrane itself, sometimes giving the impression that
they enter into direct contact with the cytoplasm. When they have
A.M.B.-6
11