316
ELVEZIO GHIRARDELLI
According to Buchner thc germ cell determinant is a small roundish
chromatic body, little differentiated, sometimes without definite
structure, or more or less vesicular or even granular in appearance. The
differences found by the various authors are almost certainly due to
fixation and to the moment at which the germ cell determinant has
been observed.
The origin of the germ cell determinant is not yet known. Theories
in this connexion differ widely and are sometimes contradictory.
Vasiljev believes it to be of cytoplasmic origin, while Elpatiewsky,
Stevens and Buchner think it has a nuclear origin. Elpatiewsky and
Stevens suppose that it may originate from the small plaques of the
nuclear net; Buchner, on the other hand, thinks it derives from the
nucleus of one of the accessory cells forming the peduncle which
connects the oocyte to the wall of the seminal pouch. According to
Vasiljev the germ cell determinant forms de novo directly after fertilization, originating from certain small grains which appear within the
cytoplasm at the vegetative pole of the egg. These grains group, unite
and form a sniall spherical body with a diameter of about 8-10 p.
In order better to understand whence and how the primordium cells
of the germinal line of chaetognaths may originate, and what is the
function of the germ cell determinant in the differentiation of the cells
of the germinal line, it is useful to outline briefly the first phases of
development. The segmentation of the eggs of Chaetognatha is total
and equal and it is the same both in Sagitta (Kuhl and Kuhl, 1965),
whose pelagic eggs are transparent since they have very little yolk, and
in Spadella cephaloptera whose eggs on the contrary are opaque, with
many vitelline grains, and are laid attached to submerged bodies,
generally seaweeds or leaves of marine phanerogams. When the eggs
begin to segment, the germ cell determinant passes undivided into one
only of the first two blastomeres which is a stem cell formed by material
which will give rise to both somatic and germinal cells (mixed blastomere) ; the other blastomere, without determinant, is a purely somatic
blastomere. Up to the fourth division, things continue in this way : the
somatic blastomere continues giving rise only to somatic cells, the
stein cell always divides to form another stem cell and a somatic
blastomere. The germ cell determinant meanwhile remains undivided
in the only mixed blastomere from which new somatic blastomeres
continue, as we have said, to separate (Fig. 18).
After the fifth division, the two cells which form from the mixed
blastomere are clearly determined : one, containing the still undivided
germ cell determinant, is the " anlage )' of the germinal cells, the other
will become the primitive endodermal cell (Elpatiewsky, 1909). During
ELVEZIO GHIRARDELLI
According to Buchner thc germ cell determinant is a small roundish
chromatic body, little differentiated, sometimes without definite
structure, or more or less vesicular or even granular in appearance. The
differences found by the various authors are almost certainly due to
fixation and to the moment at which the germ cell determinant has
been observed.
The origin of the germ cell determinant is not yet known. Theories
in this connexion differ widely and are sometimes contradictory.
Vasiljev believes it to be of cytoplasmic origin, while Elpatiewsky,
Stevens and Buchner think it has a nuclear origin. Elpatiewsky and
Stevens suppose that it may originate from the small plaques of the
nuclear net; Buchner, on the other hand, thinks it derives from the
nucleus of one of the accessory cells forming the peduncle which
connects the oocyte to the wall of the seminal pouch. According to
Vasiljev the germ cell determinant forms de novo directly after fertilization, originating from certain small grains which appear within the
cytoplasm at the vegetative pole of the egg. These grains group, unite
and form a sniall spherical body with a diameter of about 8-10 p.
In order better to understand whence and how the primordium cells
of the germinal line of chaetognaths may originate, and what is the
function of the germ cell determinant in the differentiation of the cells
of the germinal line, it is useful to outline briefly the first phases of
development. The segmentation of the eggs of Chaetognatha is total
and equal and it is the same both in Sagitta (Kuhl and Kuhl, 1965),
whose pelagic eggs are transparent since they have very little yolk, and
in Spadella cephaloptera whose eggs on the contrary are opaque, with
many vitelline grains, and are laid attached to submerged bodies,
generally seaweeds or leaves of marine phanerogams. When the eggs
begin to segment, the germ cell determinant passes undivided into one
only of the first two blastomeres which is a stem cell formed by material
which will give rise to both somatic and germinal cells (mixed blastomere) ; the other blastomere, without determinant, is a purely somatic
blastomere. Up to the fourth division, things continue in this way : the
somatic blastomere continues giving rise only to somatic cells, the
stein cell always divides to form another stem cell and a somatic
blastomere. The germ cell determinant meanwhile remains undivided
in the only mixed blastomere from which new somatic blastomeres
continue, as we have said, to separate (Fig. 18).
After the fifth division, the two cells which form from the mixed
blastomere are clearly determined : one, containing the still undivided
germ cell determinant, is the " anlage )' of the germinal cells, the other
will become the primitive endodermal cell (Elpatiewsky, 1909). During