322
ELVEZIO GHIRARDELLI
There is no doubt, therefore, that when the first germ cells form, the
determinant divides between them : the first observations with the
electron microscope, however, do not allow any statement as to
whether it divides into equal parts or not.
The various constituents of the germ cell determinant separate from
each other and distribute themselves more or less regularly in the
cytoplasm. Later the grains and small plaques tend to settle around the
nucleus of the germ cells : it would be interesting to see, in this respect,
whether they mass invariably in greater quantity around the nucleus of
two of the four primitive germ cells that are in the gastrula. In some
figures, one seems to see that, the grains are indeed in greater number
around some nuclei, which are quite similar in their appearance to the
nuclei of young oocytes in their first and second period of growth during
which the nuclear net is quite apparent. It cannot be excluded that the
origin of the germ cell determinant may be traced to these perinuclear
formations.
Actually at the first metaphase when the nuclear membrane
disappears the perinuclear formations could disperse in the cytoplasm :
these could be the grains Vasiljev saw. Later these grains could gather
at thc vegetative pole of the egg thus forming the germ cell determinant which, after dividing between the primitive germ cells, would
again give rise to the nuclear net.
The contiiiuity of the germinal line would thus be assured by the
persistence of the germ cell determinant in its various aspects. If
evidence could be found that the determinant places itself invariably in
greater quantity around the nucleus of two only of the four primitive
germ cells, it would also be proved that the differentiation of the cells
themselves is strictly connected with the dctcrminant. Owing to the
character of their nuclei, these cells would indeed be the first oocytes.
These are a t present mere hypotheses, though fascinating ones, which
need to be confirmed by further work (Ghirardelli, 1966b).
D. Fertilization
1. Fertilization in Spadella cephaloptera
The possibility of self-fertilization in Chaetognatha, which are
protandric hermaphrodites, is admitted by several authors (Stevens,
1910; Bord&s, 1920; Jagersten, 1940; Murakami, 1959; Dallot, 1966),
while others believe that fertilization is always the consequence of
mating (Grassi, 1883; Vasiljev, 1925; van Oye, 1931; John, 1933;
Vannucci and Hosoe, 1952 ; Ghirardelli, 1953d, 1954a). Cross-fertilization has, however, been seen only in a few species : in an undetermined
species of Xagitta by van Oye (1931), in Xagitta crassa by Murakami
ELVEZIO GHIRARDELLI
There is no doubt, therefore, that when the first germ cells form, the
determinant divides between them : the first observations with the
electron microscope, however, do not allow any statement as to
whether it divides into equal parts or not.
The various constituents of the germ cell determinant separate from
each other and distribute themselves more or less regularly in the
cytoplasm. Later the grains and small plaques tend to settle around the
nucleus of the germ cells : it would be interesting to see, in this respect,
whether they mass invariably in greater quantity around the nucleus of
two of the four primitive germ cells that are in the gastrula. In some
figures, one seems to see that, the grains are indeed in greater number
around some nuclei, which are quite similar in their appearance to the
nuclei of young oocytes in their first and second period of growth during
which the nuclear net is quite apparent. It cannot be excluded that the
origin of the germ cell determinant may be traced to these perinuclear
formations.
Actually at the first metaphase when the nuclear membrane
disappears the perinuclear formations could disperse in the cytoplasm :
these could be the grains Vasiljev saw. Later these grains could gather
at thc vegetative pole of the egg thus forming the germ cell determinant which, after dividing between the primitive germ cells, would
again give rise to the nuclear net.
The contiiiuity of the germinal line would thus be assured by the
persistence of the germ cell determinant in its various aspects. If
evidence could be found that the determinant places itself invariably in
greater quantity around the nucleus of two only of the four primitive
germ cells, it would also be proved that the differentiation of the cells
themselves is strictly connected with the dctcrminant. Owing to the
character of their nuclei, these cells would indeed be the first oocytes.
These are a t present mere hypotheses, though fascinating ones, which
need to be confirmed by further work (Ghirardelli, 1966b).
D. Fertilization
1. Fertilization in Spadella cephaloptera
The possibility of self-fertilization in Chaetognatha, which are
protandric hermaphrodites, is admitted by several authors (Stevens,
1910; Bord&s, 1920; Jagersten, 1940; Murakami, 1959; Dallot, 1966),
while others believe that fertilization is always the consequence of
mating (Grassi, 1883; Vasiljev, 1925; van Oye, 1931; John, 1933;
Vannucci and Hosoe, 1952 ; Ghirardelli, 1953d, 1954a). Cross-fertilization has, however, been seen only in a few species : in an undetermined
species of Xagitta by van Oye (1931), in Xagitta crassa by Murakami