37
of the oocyte are fl attened, while those surrounding the
oocyte from the sides and limiting the intraovarian zone are
oval or cubic. Most of them stain darkly, whereas basal cells
have a much lighter cytoplasm and often an irregular shape.
Only some of basal cells have a normal nucleus with a single
nucleolus; most have several (2–6) nucleoli, which may indicate enhanced synthesizing activity of the cells. Interestingly,
by light microscopy the part of the oolemma that faces the
intraovarian space appears to bear irregular projections of
varying length (Fig. 1.12C ). Ultrastructural research is
required to determine if this is a site of phagocytosis or the
basal cells of the intraovarian zone fuse with the oocyte.
Besides the change in their size and staining, enhanced
activity of the cells of the ovary is also indicated by the
presence of inclusions. For example, in Cornucopina
polymorpha (Bugulidae) prismatic cells of the ovary wall
are much larger than most other somatic cells (Fig. 1.11B ).
They have a darkly staining cytoplasm with large pale
vacuoles often arranged along the cell in a longitudinal
row. In Arctonula arctica (Romancheinidae) easily discernible dark granules may also occur in the basal cells
(including the fl attened basal cells wedged between the
oocyte and the ovary wall).
In Columnella magna (Farciminariidae) the ovary may
simultaneously contain two vitellogenic doublets (see also
below), each in its own follicle (Fig. 1.11C ). There is a separate intraovarian zone beneath each of these follicles, which
are interconnected by a loose mass of irregularly shaped
cells. Ovaries having several follicles were also found in
Corbulipora tubulifera (Cribrilinidae) and Steginoporella
perplexa (Steginoporellidae). Their walls were mostly composed of fl at follicle cells lining the surface of the oocytes.
The bases of the follicles were united by a rather loose aggregation of ovary cells (Fig. 1.12B ). Dividing oogonia and narrow intercellular spaces (presumably a small intraovarian
zone) occurred within this aggregation. The distinctive basal
cells could not, however, be distinguished in these two
species by light microscopy. The paucity of ovary cells in S .
perplexa is reminiscent of the situation in Bugula and some
other taxa with matrotrophic brooding (see Sect. 1.2.5 ).
Notably, all vitellogenic oocytes in this species have a
narrow (ca. 5 μm) peripheral cytoplasmic zone free of yolk
granules. Moreover, as the large oocyte grows it acquires a
peculiar shape – the basal part of the ovary containing follicles
with previtellogenic oocyte doublets is completely or partly
immersed in an invagination of the adjacent surface of the
vitellogenic oocyte. In other words, the ovary appears to sink
into the oocyte (Fig. 1.12D ). In one instance this invagination contained, besides the nurse cell of the leading doublet,
two previtellogenic doublets.
In Steginocellaria magnimandibulata (Cellariidae), if
there was a mature oocyte in the female gonad, the ovary
occupied up to half the volume of the zooid and was pressed
against the distal transverse wall. The intraovarian zone was
barely identifi able by the presence of fl at basal cells underlining the oocyte.
In three of the four studied representatives of the family
Lanceoporidae ( Emballotheca quadrata , Calyptotheca triangula and Parmularia smeatoni ) the basal part of the ovary
was spread fl at against the basal wall of the cystid
(Fig. 1.14B ). Early doublets were encased in follicles of fl at
cells, with the rest of the ovary wall represented by oval
cells. Large vitellogenic doublets were surrounded by fl at
follicle cells too, with prismatic cells forming the lower part
of the ovary wall (Fig. 1.14D ). Irregular basal cells (paler
than the cells of the ovary wall) and intercellular lacunae
formed the intraovarian zone, in contact with the epithelium
of the basal wall of the zooid. Basal cells underlying the
large oocyte were fl attened.
Development of Oocyte Doublets
In Scrupocellaria scabra (Candidae) two pairs of female
cells were found in the developing ovary of a zooid bud
(Fig. 1.12A ). Cell size was 37 × 20 μm in the larger (presumably oocyte) doublet and 15 × 12 μm in the smaller
(presumably oogonial). The cytoplasm of the younger
doublet cells was darker in histological sections.
In Cribrilina annulata (Cribrilinidae) a developing
ovary was recorded in a young zooid in the proximal part of
the differentiating polypide bud at a stage considerably
before inception of the feeding apparatus (Fig. 1.10A ).
Somewhat later, the ovary is displaced to the basal wall of
the cystid, presumably aided by the growing funicular network (Fig. 1.10B ). Such young ovaries consisted of a pair
of oval oogonia covered by a single layer of mesothelium,
their size being 19.9 × 13.2 μm in the doublet associated
with the polypide bud and 23 × 13 μm in the doublet in the
basal wall of the ovary. These cells had darkly staining
cytoplasm and pale nuclei.
In most studied species with reproductive pattern II,
oogonia were not found in mature ovaries. In the 40 species
where oogonia were noted, they usually numbered 1–2 (in 32
species), but occasionally there were three, as in
Schizomavella cuspidata (Bitectiporidae) and Reteporella
sp. (Phidoloporidae), four, as in Callopora lineata
(Calloporidae) and Smittina concinna (Smittinidae), or even
six, as in Dendrobeania fruticosa (Bugulidae). In two species, ovaries contained as many as 12 oogonia, as in
Hiantopora ferox (Hiantoporidae), or even 16, as in
Margaretta barbata (Margarettidae). In brooding bryozoans
with other reproductive patterns (altogether fi ve species in
which such oogonia were found), the number of single oogonia was 1–3, their size similar to the dimensions given below
(see also Tables 1.3 and 1.7 ). Only solitary oogonia were
counted and measured because early oogonial doublets are
mostly indistinguishable from early oocyte doublets.
1.2 Reproductive Patterns of Bryozoa
of the oocyte are fl attened, while those surrounding the
oocyte from the sides and limiting the intraovarian zone are
oval or cubic. Most of them stain darkly, whereas basal cells
have a much lighter cytoplasm and often an irregular shape.
Only some of basal cells have a normal nucleus with a single
nucleolus; most have several (2–6) nucleoli, which may indicate enhanced synthesizing activity of the cells. Interestingly,
by light microscopy the part of the oolemma that faces the
intraovarian space appears to bear irregular projections of
varying length (Fig. 1.12C ). Ultrastructural research is
required to determine if this is a site of phagocytosis or the
basal cells of the intraovarian zone fuse with the oocyte.
Besides the change in their size and staining, enhanced
activity of the cells of the ovary is also indicated by the
presence of inclusions. For example, in Cornucopina
polymorpha (Bugulidae) prismatic cells of the ovary wall
are much larger than most other somatic cells (Fig. 1.11B ).
They have a darkly staining cytoplasm with large pale
vacuoles often arranged along the cell in a longitudinal
row. In Arctonula arctica (Romancheinidae) easily discernible dark granules may also occur in the basal cells
(including the fl attened basal cells wedged between the
oocyte and the ovary wall).
In Columnella magna (Farciminariidae) the ovary may
simultaneously contain two vitellogenic doublets (see also
below), each in its own follicle (Fig. 1.11C ). There is a separate intraovarian zone beneath each of these follicles, which
are interconnected by a loose mass of irregularly shaped
cells. Ovaries having several follicles were also found in
Corbulipora tubulifera (Cribrilinidae) and Steginoporella
perplexa (Steginoporellidae). Their walls were mostly composed of fl at follicle cells lining the surface of the oocytes.
The bases of the follicles were united by a rather loose aggregation of ovary cells (Fig. 1.12B ). Dividing oogonia and narrow intercellular spaces (presumably a small intraovarian
zone) occurred within this aggregation. The distinctive basal
cells could not, however, be distinguished in these two
species by light microscopy. The paucity of ovary cells in S .
perplexa is reminiscent of the situation in Bugula and some
other taxa with matrotrophic brooding (see Sect. 1.2.5 ).
Notably, all vitellogenic oocytes in this species have a
narrow (ca. 5 μm) peripheral cytoplasmic zone free of yolk
granules. Moreover, as the large oocyte grows it acquires a
peculiar shape – the basal part of the ovary containing follicles
with previtellogenic oocyte doublets is completely or partly
immersed in an invagination of the adjacent surface of the
vitellogenic oocyte. In other words, the ovary appears to sink
into the oocyte (Fig. 1.12D ). In one instance this invagination contained, besides the nurse cell of the leading doublet,
two previtellogenic doublets.
In Steginocellaria magnimandibulata (Cellariidae), if
there was a mature oocyte in the female gonad, the ovary
occupied up to half the volume of the zooid and was pressed
against the distal transverse wall. The intraovarian zone was
barely identifi able by the presence of fl at basal cells underlining the oocyte.
In three of the four studied representatives of the family
Lanceoporidae ( Emballotheca quadrata , Calyptotheca triangula and Parmularia smeatoni ) the basal part of the ovary
was spread fl at against the basal wall of the cystid
(Fig. 1.14B ). Early doublets were encased in follicles of fl at
cells, with the rest of the ovary wall represented by oval
cells. Large vitellogenic doublets were surrounded by fl at
follicle cells too, with prismatic cells forming the lower part
of the ovary wall (Fig. 1.14D ). Irregular basal cells (paler
than the cells of the ovary wall) and intercellular lacunae
formed the intraovarian zone, in contact with the epithelium
of the basal wall of the zooid. Basal cells underlying the
large oocyte were fl attened.
Development of Oocyte Doublets
In Scrupocellaria scabra (Candidae) two pairs of female
cells were found in the developing ovary of a zooid bud
(Fig. 1.12A ). Cell size was 37 × 20 μm in the larger (presumably oocyte) doublet and 15 × 12 μm in the smaller
(presumably oogonial). The cytoplasm of the younger
doublet cells was darker in histological sections.
In Cribrilina annulata (Cribrilinidae) a developing
ovary was recorded in a young zooid in the proximal part of
the differentiating polypide bud at a stage considerably
before inception of the feeding apparatus (Fig. 1.10A ).
Somewhat later, the ovary is displaced to the basal wall of
the cystid, presumably aided by the growing funicular network (Fig. 1.10B ). Such young ovaries consisted of a pair
of oval oogonia covered by a single layer of mesothelium,
their size being 19.9 × 13.2 μm in the doublet associated
with the polypide bud and 23 × 13 μm in the doublet in the
basal wall of the ovary. These cells had darkly staining
cytoplasm and pale nuclei.
In most studied species with reproductive pattern II,
oogonia were not found in mature ovaries. In the 40 species
where oogonia were noted, they usually numbered 1–2 (in 32
species), but occasionally there were three, as in
Schizomavella cuspidata (Bitectiporidae) and Reteporella
sp. (Phidoloporidae), four, as in Callopora lineata
(Calloporidae) and Smittina concinna (Smittinidae), or even
six, as in Dendrobeania fruticosa (Bugulidae). In two species, ovaries contained as many as 12 oogonia, as in
Hiantopora ferox (Hiantoporidae), or even 16, as in
Margaretta barbata (Margarettidae). In brooding bryozoans
with other reproductive patterns (altogether fi ve species in
which such oogonia were found), the number of single oogonia was 1–3, their size similar to the dimensions given below
(see also Tables 1.3 and 1.7 ). Only solitary oogonia were
counted and measured because early oogonial doublets are
mostly indistinguishable from early oocyte doublets.
1.2 Reproductive Patterns of Bryozoa