300
ELVEZIO GISIRARDELLI
the cells of the cubic epithelium which has not yet differentiated; on
the left side of the same figure are two oocytes, the smallest of which
is connected with a cell that can easily be recognized as belonging to
the cubic epithelium ; in the larger oocyte the process of transformation
is still evident. Proceeding from the smallest oocyte a t the right side to
the largest, one can observe different stages of the process through
which the apparatus of fertilization is developed ; this apparatus may
reasonably be considered as deriving from the cells of the crescent.
I n Sagitta minima the structure of the fertilization apparatus seems
to be a little more complex as there appear to be a number of fertilization cells greater than those in 8. bipunctata. As to the support cells
after fertilization, all workers agree that they degenerate. Opinions
differ as to the point in the ovary where the second accessory cell
degenerates. Buchner (lSlOb), Bordks (1920) and Stevens (1903)
herself, believed that this cell degenerates in the ovular cytoplasm ;
later Stevens (1910) and Vasiljev (1925) ruled this out. It happens
sometimes that the nucleus of this cell is seen to have a pycnotic
character that might be a sign of degeneration, but it has never been
possible to observe further stages of the degenerative process. Though
I feel rather inclined to think that tile second accessory cell does not
degenerate inside the egg as some believe, the matter has some importance also for the question of the origin of the germ cell determinant :
Buchner deemed that it derives from the degeneration of the fertilization
accessory cell in contact with the egg.
3. Thk female reproductive apparatus in Spadella
Only Grassi (1883) and more recently Vasiljev (1925) and John
(1933) have done adequate research on the anatomy of the female
gonad in Spadella.
As in Xagitta, so in Xpadella the ovaries are placed in the coelom of
the trunk, directly before the caudal septum. They are generally 1 mm
long but in spring may become longer, their cephalic extremity reaching
the region of the neck a t the level of the ciliary loop. They are wrapped
in a thin endothelium which connects them with the walls of the body.
Each ovary contains a few dozen eggs of which a variable quantity has
attained the maximum size. During summer the two ovaries contain
about ten mature eggs, but in springtime there may be more than
twenty, while in autumn they decrease to two or even one only in each
ovary. We shall return to this point when dealing with sexual maturity
I n the median and posterior region, one normally finds the oocytes
in the first periods of growth mixed with those which have completed
(p. 349).
ELVEZIO GISIRARDELLI
the cells of the cubic epithelium which has not yet differentiated; on
the left side of the same figure are two oocytes, the smallest of which
is connected with a cell that can easily be recognized as belonging to
the cubic epithelium ; in the larger oocyte the process of transformation
is still evident. Proceeding from the smallest oocyte a t the right side to
the largest, one can observe different stages of the process through
which the apparatus of fertilization is developed ; this apparatus may
reasonably be considered as deriving from the cells of the crescent.
I n Sagitta minima the structure of the fertilization apparatus seems
to be a little more complex as there appear to be a number of fertilization cells greater than those in 8. bipunctata. As to the support cells
after fertilization, all workers agree that they degenerate. Opinions
differ as to the point in the ovary where the second accessory cell
degenerates. Buchner (lSlOb), Bordks (1920) and Stevens (1903)
herself, believed that this cell degenerates in the ovular cytoplasm ;
later Stevens (1910) and Vasiljev (1925) ruled this out. It happens
sometimes that the nucleus of this cell is seen to have a pycnotic
character that might be a sign of degeneration, but it has never been
possible to observe further stages of the degenerative process. Though
I feel rather inclined to think that tile second accessory cell does not
degenerate inside the egg as some believe, the matter has some importance also for the question of the origin of the germ cell determinant :
Buchner deemed that it derives from the degeneration of the fertilization
accessory cell in contact with the egg.
3. Thk female reproductive apparatus in Spadella
Only Grassi (1883) and more recently Vasiljev (1925) and John
(1933) have done adequate research on the anatomy of the female
gonad in Spadella.
As in Xagitta, so in Xpadella the ovaries are placed in the coelom of
the trunk, directly before the caudal septum. They are generally 1 mm
long but in spring may become longer, their cephalic extremity reaching
the region of the neck a t the level of the ciliary loop. They are wrapped
in a thin endothelium which connects them with the walls of the body.
Each ovary contains a few dozen eggs of which a variable quantity has
attained the maximum size. During summer the two ovaries contain
about ten mature eggs, but in springtime there may be more than
twenty, while in autumn they decrease to two or even one only in each
ovary. We shall return to this point when dealing with sexual maturity
I n the median and posterior region, one normally finds the oocytes
in the first periods of growth mixed with those which have completed
(p. 349).