SOME ASPECTS O F THE BIOLOGY O F T H E CHAETOGNATHS
293
epithelial cells disposed in a sort of half-moon with its concavity facing
towards the intestine. I n the middle of the half-moon the two epithelial
layers diverge from each other and a small canal runs through the
tubular space so formed, the wall of which has a syncytial character.
This canal is the sperm-oviduct of Grassi.
I n the concavity of the half-moon, the oocytes are inserted by
means of a sort of peduncle made of two cells peculiarly differentiated :
the lower club-shaped cell penetrates with its end among the cylindrical
cells of the seminal receptacle. The upper kidney-shaped cell rests with
its concave part on the convex one of the lower cell and is in strict
connexion with the egg : it even seems partially immersed in its cytoplasm (Buchner, 1910a,b). The purpose of these cells is to allow the
passage of the spermatozoon from the seminal pouch to the oocyte.
The penetration of the spermatozoon into the egg cell occurs, therefore,
while the oocyte is still in the ovary. When the sperm has penetrated,
emission of the first polar body occurs. Immediately there follows thc
second maturity division with the emission of the second polar body.
Ruchner believes that the innermost cell penetrates into the egg
together with the spermatozoon and that from it the germ cell
determinant originates, but we shall deal with this later.
1. The female reproductive apparatus in Sagitta
The researches I have been able t o carry out on material coming
from the gulf of Naples and the Bay of Villefranche have enabled me
to confirm to a great extent the results of the works of earlier authors,
especially of Stevens, and to correct some mistakes found in other
works concerning the anatomy of the female genital apparatus in
chaetognaths and particularly in Xagitta which is the genus which was
more widely studied. Stevens mostly studied Sagitta elegans Verrill and
X . bipunctata. I have had the opportunity also of observing, besides
the latter species, S. in$ata and Xagitta minim,a Grassi.
Each ovary and seminal pouch is covered by a thin endothelium
which is continuous with the lining of the general cavity of the body in
the region of the lateral line. It is difficult to see this membrane in
section, and in any case it can be observed better in life with the phase
contrast microscope. Thus each ovary is attached to the body wall
laterally by means of a thin mesentery. Anteriorly the mesentery forms
a triangular lamina which ends at the level of the ventral ganglion. The
ovary appears t o be formed by a mass of oocytes in various stages of
development and by support cells. Those oocytes which are in a more
advanced stage of vitellogenesis are placed in the middle, i.e. close to
that side of the ovary facing the intestine. The less developed oocytes,
293
epithelial cells disposed in a sort of half-moon with its concavity facing
towards the intestine. I n the middle of the half-moon the two epithelial
layers diverge from each other and a small canal runs through the
tubular space so formed, the wall of which has a syncytial character.
This canal is the sperm-oviduct of Grassi.
I n the concavity of the half-moon, the oocytes are inserted by
means of a sort of peduncle made of two cells peculiarly differentiated :
the lower club-shaped cell penetrates with its end among the cylindrical
cells of the seminal receptacle. The upper kidney-shaped cell rests with
its concave part on the convex one of the lower cell and is in strict
connexion with the egg : it even seems partially immersed in its cytoplasm (Buchner, 1910a,b). The purpose of these cells is to allow the
passage of the spermatozoon from the seminal pouch to the oocyte.
The penetration of the spermatozoon into the egg cell occurs, therefore,
while the oocyte is still in the ovary. When the sperm has penetrated,
emission of the first polar body occurs. Immediately there follows thc
second maturity division with the emission of the second polar body.
Ruchner believes that the innermost cell penetrates into the egg
together with the spermatozoon and that from it the germ cell
determinant originates, but we shall deal with this later.
1. The female reproductive apparatus in Sagitta
The researches I have been able t o carry out on material coming
from the gulf of Naples and the Bay of Villefranche have enabled me
to confirm to a great extent the results of the works of earlier authors,
especially of Stevens, and to correct some mistakes found in other
works concerning the anatomy of the female genital apparatus in
chaetognaths and particularly in Xagitta which is the genus which was
more widely studied. Stevens mostly studied Sagitta elegans Verrill and
X . bipunctata. I have had the opportunity also of observing, besides
the latter species, S. in$ata and Xagitta minim,a Grassi.
Each ovary and seminal pouch is covered by a thin endothelium
which is continuous with the lining of the general cavity of the body in
the region of the lateral line. It is difficult to see this membrane in
section, and in any case it can be observed better in life with the phase
contrast microscope. Thus each ovary is attached to the body wall
laterally by means of a thin mesentery. Anteriorly the mesentery forms
a triangular lamina which ends at the level of the ventral ganglion. The
ovary appears t o be formed by a mass of oocytes in various stages of
development and by support cells. Those oocytes which are in a more
advanced stage of vitellogenesis are placed in the middle, i.e. close to
that side of the ovary facing the intestine. The less developed oocytes,