310
beneath the frontal membrane in this species. The ovary differentiates in the somatic peritoneum of one of the lateral
walls in the proximal region of the zooid, at the confl uence of
several funicular cords. The ovary wall consists of follicle
cells of peritoneal origin, enveloping the oogonia and
oocytes, and there is a so-called “subovarian space” between
the ovary and cystid epidermis. This is confl uent with the
lumina of the funicular cords and is lined by special “basal
cells” [of peritoneal origin also]. In the peripheral germinal
zone, follicle cells completely surround the oogonia and
early previtellogenic oocytes, which remain connected by
cytoplasmic bridges after oogonial divisions. Hageman
suggested that follicle cells may regulate vitellogenesis by
compartmentalizing the ovary, synchronizing oocyte differentiation and transporting low-molecular- weight precursors
or metabolites. Interestingly, it was also shown that follicle
cells phagocytose degenerating oocytes. In the central growing zone, the follicle epithelium is not complete basally, and
oocytes at various stages of vitellogenesis are in contact with
the subovarian space. Additionally, early vitellogenic oocytes
share gap junctions with the overlying follicle cells. Basal
cells secret yolk precursors that are endocytosed by the
oocytes from the “subovarian space” (heterosynthetic source
of yolk), and incorporated into yolk granules (autosynthetic
source) in the oocytes. Later the vitelline envelope forms at
the oolemma. Further, oocytes move into the apical zone,
forming microvilli on that part of the surface exposed to the
coelom. A similar situation was described by Hughes ( 1987 )
in Celleporella hyalina (see below). Upon breakdown of the
germinal vesicle, the mature oocytes ovulate, with up to 25
accumulating in the zooid cavity. Ovulated eggs and spermatozeugmata are transported to the base of the lophophore by
the ciliated pharyngeal gutter that forms at the onset of spermatogenesis. During the male phase of reproduction, sperm
are transported to the lumina of the dorsomedial tentacles by
the ciliary tracts lateral to the gutter. During the female
phase, the eggs are transported along the ciliated fl oor of the
gutter into the base of the two- chambered intertentacular
organ. It forms at the onset of oogenesis from the epithelium
of the two dorsomedial tentacles and is completed in approximately 2 days. This process does not involve polypide
replacement.
Hayward ( 1983 ) reviewed bryozoan oogenesis. Despite
the relatively paucity of papers consulted, he correctly
pointed out the existing imbalance in research on bryozoan
reproduction. Whereas spermatogenesis, embryology and
larval morphology had attracted considerable attention, the
processes of oogenesis and fertilization were relatively
understudied. Even today, the great variety of reproductive
patterns and associated morphologies recorded in the relatively restricted number of species studied makes the current
picture very confused, showing that further research is
urgently needed to verify many of the data and conclusions
made by earlier authors. Hayward stressed the most important problems and questions, including variations in the
site of origin and fi nal position of ovaries, the temporary
relationship between the appearance and development of the
polypide bud and early ovary, the actual source of ovarian
cells and the poorly known cytology of the ovary. He presented sections of the female gonad of Alcyonidium hirsutum
for comparative purposes.
Jebram ( 1985 ) described the ctenostome Panolicella
nutans as a protandrous hermaphrodite. [Actually, this bryozoan was described a little earlier by Kayser ( 1984 ), who
gave some information on reproduction.] Spermatogenic tissue develops on the basal wall in the proximal part of the
zooid and the ovary on the funiculus at the end of the caecum. The ovary contains several oocytes that mature sequentially and exit through the supraneural pore. The maternal
zooid simultaneously broods 4–5 embryos that are externally
attached to the maternal cystid wall by the sticky fertilization
envelope. Embryos of different ages are sometimes positioned chaotically, but often form a line in which the oldest
embryo is the lowest. The distalmost egg can be withdrawn
and sheltered inside the vestibulum during polypide retraction. Judging by the time of appearance of the “perivitellar
membrane” [fertilization envelope], Jebram suggested that
fertilization takes place during egg release.
Hughes ( 1987 ) investigated the reproductive biology and
anatomy of Celleporella hyalina . He carefully described formation of both male and female autozooidal polymorphs,
and presented the results of the light-microscopic and ultrastructural studies of their gonads. The coelomic cavity of
males is largely fi lled with spermatogenic tissue. The ovary
is positioned on the basal wall of the female, while the distal
part of the zooid contains cells providing placental nutrition
[embryophore]. Oocytes develop in doublets, being surrounded by squamous follicle cells in at least the early vitellogenetic stage. The source of nutrients for the early stages
of vitellogenesis is unclear (and may be connected with the
activity of the nurse cell), since there is no pinocytosis until
the enlarged leading oocyte breaks through the thin follicular
layer and its surface is partially exposed to the maternal coelom. The oolemma in the exposed region becomes microvillous, possibly allowing nutrient uptake directly from the
coelomic fl uid. Hughes suggested that the source of nutrients
could be certain peritoneal cells with numerous yolk-like
inclusions, presumably representing nutrient-storage cells.
In fact, these cells belong to the funicular tissue forming part
of the embryophore. Sperm heads were found among ovarian
cells, and syngamy is suggested to occur during the previtellogenic or vitellogenetic phase of development. However,
the route of the sperm to the ovary was not explained, since
Hughes thought that the rudimentary female polypide could
not protrude. The mature macrolecithal oocyte fi lls most of
the coelom, accumulating many large yolk inclusions.
Appendices
beneath the frontal membrane in this species. The ovary differentiates in the somatic peritoneum of one of the lateral
walls in the proximal region of the zooid, at the confl uence of
several funicular cords. The ovary wall consists of follicle
cells of peritoneal origin, enveloping the oogonia and
oocytes, and there is a so-called “subovarian space” between
the ovary and cystid epidermis. This is confl uent with the
lumina of the funicular cords and is lined by special “basal
cells” [of peritoneal origin also]. In the peripheral germinal
zone, follicle cells completely surround the oogonia and
early previtellogenic oocytes, which remain connected by
cytoplasmic bridges after oogonial divisions. Hageman
suggested that follicle cells may regulate vitellogenesis by
compartmentalizing the ovary, synchronizing oocyte differentiation and transporting low-molecular- weight precursors
or metabolites. Interestingly, it was also shown that follicle
cells phagocytose degenerating oocytes. In the central growing zone, the follicle epithelium is not complete basally, and
oocytes at various stages of vitellogenesis are in contact with
the subovarian space. Additionally, early vitellogenic oocytes
share gap junctions with the overlying follicle cells. Basal
cells secret yolk precursors that are endocytosed by the
oocytes from the “subovarian space” (heterosynthetic source
of yolk), and incorporated into yolk granules (autosynthetic
source) in the oocytes. Later the vitelline envelope forms at
the oolemma. Further, oocytes move into the apical zone,
forming microvilli on that part of the surface exposed to the
coelom. A similar situation was described by Hughes ( 1987 )
in Celleporella hyalina (see below). Upon breakdown of the
germinal vesicle, the mature oocytes ovulate, with up to 25
accumulating in the zooid cavity. Ovulated eggs and spermatozeugmata are transported to the base of the lophophore by
the ciliated pharyngeal gutter that forms at the onset of spermatogenesis. During the male phase of reproduction, sperm
are transported to the lumina of the dorsomedial tentacles by
the ciliary tracts lateral to the gutter. During the female
phase, the eggs are transported along the ciliated fl oor of the
gutter into the base of the two- chambered intertentacular
organ. It forms at the onset of oogenesis from the epithelium
of the two dorsomedial tentacles and is completed in approximately 2 days. This process does not involve polypide
replacement.
Hayward ( 1983 ) reviewed bryozoan oogenesis. Despite
the relatively paucity of papers consulted, he correctly
pointed out the existing imbalance in research on bryozoan
reproduction. Whereas spermatogenesis, embryology and
larval morphology had attracted considerable attention, the
processes of oogenesis and fertilization were relatively
understudied. Even today, the great variety of reproductive
patterns and associated morphologies recorded in the relatively restricted number of species studied makes the current
picture very confused, showing that further research is
urgently needed to verify many of the data and conclusions
made by earlier authors. Hayward stressed the most important problems and questions, including variations in the
site of origin and fi nal position of ovaries, the temporary
relationship between the appearance and development of the
polypide bud and early ovary, the actual source of ovarian
cells and the poorly known cytology of the ovary. He presented sections of the female gonad of Alcyonidium hirsutum
for comparative purposes.
Jebram ( 1985 ) described the ctenostome Panolicella
nutans as a protandrous hermaphrodite. [Actually, this bryozoan was described a little earlier by Kayser ( 1984 ), who
gave some information on reproduction.] Spermatogenic tissue develops on the basal wall in the proximal part of the
zooid and the ovary on the funiculus at the end of the caecum. The ovary contains several oocytes that mature sequentially and exit through the supraneural pore. The maternal
zooid simultaneously broods 4–5 embryos that are externally
attached to the maternal cystid wall by the sticky fertilization
envelope. Embryos of different ages are sometimes positioned chaotically, but often form a line in which the oldest
embryo is the lowest. The distalmost egg can be withdrawn
and sheltered inside the vestibulum during polypide retraction. Judging by the time of appearance of the “perivitellar
membrane” [fertilization envelope], Jebram suggested that
fertilization takes place during egg release.
Hughes ( 1987 ) investigated the reproductive biology and
anatomy of Celleporella hyalina . He carefully described formation of both male and female autozooidal polymorphs,
and presented the results of the light-microscopic and ultrastructural studies of their gonads. The coelomic cavity of
males is largely fi lled with spermatogenic tissue. The ovary
is positioned on the basal wall of the female, while the distal
part of the zooid contains cells providing placental nutrition
[embryophore]. Oocytes develop in doublets, being surrounded by squamous follicle cells in at least the early vitellogenetic stage. The source of nutrients for the early stages
of vitellogenesis is unclear (and may be connected with the
activity of the nurse cell), since there is no pinocytosis until
the enlarged leading oocyte breaks through the thin follicular
layer and its surface is partially exposed to the maternal coelom. The oolemma in the exposed region becomes microvillous, possibly allowing nutrient uptake directly from the
coelomic fl uid. Hughes suggested that the source of nutrients
could be certain peritoneal cells with numerous yolk-like
inclusions, presumably representing nutrient-storage cells.
In fact, these cells belong to the funicular tissue forming part
of the embryophore. Sperm heads were found among ovarian
cells, and syngamy is suggested to occur during the previtellogenic or vitellogenetic phase of development. However,
the route of the sperm to the ovary was not explained, since
Hughes thought that the rudimentary female polypide could
not protrude. The mature macrolecithal oocyte fi lls most of
the coelom, accumulating many large yolk inclusions.
Appendices