38
The diameter of early oogonia in mature ovaries of species with reproductive pattern II is rather stable, varying only
from 6 to 8 μm (nucleus 4–6 μm). They are single vesicular
or oval cells with darkly staining cytoplasm and a large
nucleus. Growing and premitotic oogonia were, as a rule,
oval, with their cytoplasm and nuclei usually staining more
intensely in histological sections than those of oocyte doublets. These oogonia generally had a mean diameter of
13.7 μm but some could be as large as 22–23 μm. In all
instances the proportion between cell size and nucleus size
may vary – large oogonia can have small nuclei and vice
versa (see Table 1.3 ).
In the cheilostomes studied, the number of doublets,
whether oogonial or oocyte, varied greatly. In 17 species a
single doublet was found, in 32 species there were not
more than 2, in 26 species up to 3, and in 16 up to 4. Up to
fi ve doublets were noted in Tegella unicornis and
Valdemunitella lata (Calloporidae), up to six doublets in
Hippoporella hippopus (Hippoporidridae), and up to seven
doublets in Corbulipora tubulifera (Cribrilinidae),
Arachnopusia
unicornis
(Arachnopusiidae)
and
Quadriscutella papillata (Phorioppniidae). The ovaries of
Cribrilina macropunctata contained up to eight doublets,
those of Nematofl ustra fl agellata (Flustridae) and C. annulata (Cribrilinidae) (Fig. 1.10D ) up to ten and Puellina radiata (Cribrilinidae) had up to 12. As many as 25 (!) doublets
were found in one of the ovaries in Margaretta barbata
(Margarettidae) (Fig. 1.15A ). All of the mature ovaries
examined in this species contained more than 10 doublets
(see Table 1.7 ).
Because the number of ovaries studied varied from species to species, it cannot be stated with certainty what the
maximum is for these species. Therefore, these results should
be considered as preliminary. Besides, early oocyte doublets
can be differentiated from oogonial doublets at the level of
light microscopy only if the sperm head is found in one of
the siblings (Figs. 1.13 inset, and 1.35C ). It is therefore probable that some oocyte and oogonial doublets were counted
together.
Minimum cell size in young doublets varied from
6.6 × 5.5 μm to 7 × 6 μm. Further growth of oocyte doublets
was near-synchronous up to the beginning of the vitellogenic
phase (Fig. 1.13 inset), though sometimes nurse cells (in
Schizomavella cuspidata : Bitectiporidae) or their nuclei (in
Escharella immersa and Exochella sp.: Romancheinidae)
could be somewhat larger than oocytes and their nuclei. The
diameter that oocytes must achieve for vitellogenesis to start
(Fig. 1.13C ) depends both on the fi nal size of mature oocytes
(as a rule, the larger they are, the larger are early vitellogenic
oocytes) and the time of ovulation of the leading oocyte (see
above). In most of the species studied (37 of 47 in which
early vitellogenesis was recorded), the accumulation of yolk
granules began when oocyte diameter was in the range of
25–30 to 67.5 μm (see Table 1.4 ). It should be noted that
yolk granules in early vitellogenic oocytes are not only less
numerous but also smaller than those in mature oocytes. As
oocytes mature, the number of granules and their size
increases considerably.
The ovaries of most of the species studied with reproductive pattern II have only one vitellogenic doublet at a time.
During ovulation, and for some time after, there is no
vitellogenic oocyte in the ovary; after the start of vitellogenesis,
its role is taken up by the next oldest previtellogenic doublet.
Eighteen species were exceptions, having more than one
vitellogenic doublet in the ovary – up to two in 11 species
(Figs. 1.11C and 1.13B, C ), up to three in six species and up
to six in Quadriscutella papillata (Fig. 1.14C ). Interestingly,
two species with more than one vitellogenic doublet in the
ovary were found in each of the families Flustridae,
Smittinidae, Bitectiporidae and Celleporidae and three such
species in each of Bryocryptellidae and Celleporidae. The
ovary can contain both the vitellogenic and previtellogenic
doublet(s) in such species. In Eurystomella foraminigera
(Eurystomellidae) and Bostrychopora dentata (Smittinidae),
all doublets encountered in the ovary (up to 3) were at different stages of vitellogenesis. Moreover, in the two latter species yolk granules were found in both the oocytes and the
nurse cells (see Table 1.7 ).
At the beginning of vitellogenesis, nurse cells are usually
smaller than their sibling oocytes. In some species their size
at this time is either the same as the oocyte ( Turbicellepora
crenulata , Celleporidae) or a little smaller. Notably, the later
an oocyte doublet undergoes vitellogenesis, the greater the
difference between the siblings. For example, in one of the
early vitellogenic doublets of Hippoporina propinqua
(Bitectiporidae), the size of the oocyte and its nurse cell was
almost the same (35 × 15 μm and 34 × 15 μm, respectively),
while in the other they differed strongly, being 75 × 55 μm
and 55 × 30 μm. Thus, the beginning of asynchronous
growth of the siblings is not connected with the start of
vitellogenesis. Besides, in Mucropetraliella ellerii
(Petraliellidae) and Petralia undata (Petraliidae) the nurse
cells in early vitellogenic doublets were in some instances
smaller than those in previtellogenic doublets, perhaps
resulting from intrinsic differences at the beginning of the
vitellogenic phase. In Dimetopia cornuta (Bugulidae), one
of the smallest nurse cells in the vitellogenic doublets studied was the sibling of the largest oocyte found in this species
(see Table 1.5 ).
In most species studied mature (ready-to-ovulate or justovulated) macrolecithal oocytes are plasmalecithal, with
yolk granules evenly distributed throughout the cytoplasm
(Figs. 1.10C, D , 1.11 , 1.12C, D , 1.13B, D , 1.14C, D and
1.15 ). Oocytes are telolecithal, with yolk granules occupying
some or often most of the cytoplasm and the rest of it being
yolkless, in Dendrobeania fruticosa , Porella proboscidea
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
The diameter of early oogonia in mature ovaries of species with reproductive pattern II is rather stable, varying only
from 6 to 8 μm (nucleus 4–6 μm). They are single vesicular
or oval cells with darkly staining cytoplasm and a large
nucleus. Growing and premitotic oogonia were, as a rule,
oval, with their cytoplasm and nuclei usually staining more
intensely in histological sections than those of oocyte doublets. These oogonia generally had a mean diameter of
13.7 μm but some could be as large as 22–23 μm. In all
instances the proportion between cell size and nucleus size
may vary – large oogonia can have small nuclei and vice
versa (see Table 1.3 ).
In the cheilostomes studied, the number of doublets,
whether oogonial or oocyte, varied greatly. In 17 species a
single doublet was found, in 32 species there were not
more than 2, in 26 species up to 3, and in 16 up to 4. Up to
fi ve doublets were noted in Tegella unicornis and
Valdemunitella lata (Calloporidae), up to six doublets in
Hippoporella hippopus (Hippoporidridae), and up to seven
doublets in Corbulipora tubulifera (Cribrilinidae),
Arachnopusia
unicornis
(Arachnopusiidae)
and
Quadriscutella papillata (Phorioppniidae). The ovaries of
Cribrilina macropunctata contained up to eight doublets,
those of Nematofl ustra fl agellata (Flustridae) and C. annulata (Cribrilinidae) (Fig. 1.10D ) up to ten and Puellina radiata (Cribrilinidae) had up to 12. As many as 25 (!) doublets
were found in one of the ovaries in Margaretta barbata
(Margarettidae) (Fig. 1.15A ). All of the mature ovaries
examined in this species contained more than 10 doublets
(see Table 1.7 ).
Because the number of ovaries studied varied from species to species, it cannot be stated with certainty what the
maximum is for these species. Therefore, these results should
be considered as preliminary. Besides, early oocyte doublets
can be differentiated from oogonial doublets at the level of
light microscopy only if the sperm head is found in one of
the siblings (Figs. 1.13 inset, and 1.35C ). It is therefore probable that some oocyte and oogonial doublets were counted
together.
Minimum cell size in young doublets varied from
6.6 × 5.5 μm to 7 × 6 μm. Further growth of oocyte doublets
was near-synchronous up to the beginning of the vitellogenic
phase (Fig. 1.13 inset), though sometimes nurse cells (in
Schizomavella cuspidata : Bitectiporidae) or their nuclei (in
Escharella immersa and Exochella sp.: Romancheinidae)
could be somewhat larger than oocytes and their nuclei. The
diameter that oocytes must achieve for vitellogenesis to start
(Fig. 1.13C ) depends both on the fi nal size of mature oocytes
(as a rule, the larger they are, the larger are early vitellogenic
oocytes) and the time of ovulation of the leading oocyte (see
above). In most of the species studied (37 of 47 in which
early vitellogenesis was recorded), the accumulation of yolk
granules began when oocyte diameter was in the range of
25–30 to 67.5 μm (see Table 1.4 ). It should be noted that
yolk granules in early vitellogenic oocytes are not only less
numerous but also smaller than those in mature oocytes. As
oocytes mature, the number of granules and their size
increases considerably.
The ovaries of most of the species studied with reproductive pattern II have only one vitellogenic doublet at a time.
During ovulation, and for some time after, there is no
vitellogenic oocyte in the ovary; after the start of vitellogenesis,
its role is taken up by the next oldest previtellogenic doublet.
Eighteen species were exceptions, having more than one
vitellogenic doublet in the ovary – up to two in 11 species
(Figs. 1.11C and 1.13B, C ), up to three in six species and up
to six in Quadriscutella papillata (Fig. 1.14C ). Interestingly,
two species with more than one vitellogenic doublet in the
ovary were found in each of the families Flustridae,
Smittinidae, Bitectiporidae and Celleporidae and three such
species in each of Bryocryptellidae and Celleporidae. The
ovary can contain both the vitellogenic and previtellogenic
doublet(s) in such species. In Eurystomella foraminigera
(Eurystomellidae) and Bostrychopora dentata (Smittinidae),
all doublets encountered in the ovary (up to 3) were at different stages of vitellogenesis. Moreover, in the two latter species yolk granules were found in both the oocytes and the
nurse cells (see Table 1.7 ).
At the beginning of vitellogenesis, nurse cells are usually
smaller than their sibling oocytes. In some species their size
at this time is either the same as the oocyte ( Turbicellepora
crenulata , Celleporidae) or a little smaller. Notably, the later
an oocyte doublet undergoes vitellogenesis, the greater the
difference between the siblings. For example, in one of the
early vitellogenic doublets of Hippoporina propinqua
(Bitectiporidae), the size of the oocyte and its nurse cell was
almost the same (35 × 15 μm and 34 × 15 μm, respectively),
while in the other they differed strongly, being 75 × 55 μm
and 55 × 30 μm. Thus, the beginning of asynchronous
growth of the siblings is not connected with the start of
vitellogenesis. Besides, in Mucropetraliella ellerii
(Petraliellidae) and Petralia undata (Petraliidae) the nurse
cells in early vitellogenic doublets were in some instances
smaller than those in previtellogenic doublets, perhaps
resulting from intrinsic differences at the beginning of the
vitellogenic phase. In Dimetopia cornuta (Bugulidae), one
of the smallest nurse cells in the vitellogenic doublets studied was the sibling of the largest oocyte found in this species
(see Table 1.5 ).
In most species studied mature (ready-to-ovulate or justovulated) macrolecithal oocytes are plasmalecithal, with
yolk granules evenly distributed throughout the cytoplasm
(Figs. 1.10C, D , 1.11 , 1.12C, D , 1.13B, D , 1.14C, D and
1.15 ). Oocytes are telolecithal, with yolk granules occupying
some or often most of the cytoplasm and the rest of it being
yolkless, in Dendrobeania fruticosa , Porella proboscidea
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis