18
During the growth phase the oocyte increases in volume
ca. 6,000–8,000-fold. Towards the end of the vitellogenic
phase, oocytes lose contact with the subovarian space and
move into the centro-apical ovulatory zone where they are
partially exposed to the zooidal cavity and their oolemma
forms numerous microvilli. Hageman ( 1983 ) was not sure if
endocytosis occurred between their bases, however.
Following vitellogenesis and some time after the destruction
of the nuclear envelope oocytes ovulate, accumulating before
spawning in the coelom of the maternal zooid in groups of
20–30. Mature primary oocytes are shaped as a biconcave
elongated disc 85.8–101 μm in diameter.
Fertilization in malacostegans occurs during or shortly
after ovulation (Temkin 1994 , 1996 ). Sperm enter the coelom of fertile zooids via the intertentacular organ, through
which eggs are also spawned.
My data on oogenesis in Electra pilosa (Fig. 1.1A, B ) are
in generally good agreement with the results of Hageman
( 1983 ), as well as the earlier fi ndings of Calvet ( 1900 ) and
Bonnevie ( 1907 ). In this species the ovary is situated in the
proximal half of the autozooid, generally on the basal wall,
often adjoining one of the lateral walls (Figs. 1.2C, D
and 1.3B ). In some cases, however, only part of the gonad is
on the basal wall, the rest of it being suspended on funicular
cords in the zooidal cavity (Figs. 1.2A, B and 1.3A ). The
mature ovary often occupies a rather considerable area of the
basal-wall surface (Fig. 1.3B ).
The ovary consists of oogonia and oocytes surrounded by
the follicle cells (Figs. 1.2 and 1.3A ). The latter have an
irregular or fl attened oval shape and a size of 2.5–4.0 μm; the
staining of their cytoplasm and the nucleus is, in general,
similar to that of the epithelial cells of the zooid walls.
Intercellular spaces are mostly confi ned to the basal part of
the ovary; they are presumably parts of the subovarian space
revealed by Hageman ( 1983 ).
Oocytes of different size in the ovary are situated in three
main zones – germinal (peripheral), growth (subperipheral)
and ovulatory (central or subcentral), defi ned by their size
and appearance. [Between these zones are oocytes of intermediate size and the borders between them can barely be seen
by light microscopy (cf. Bonnevie 1907 , pl. 35, fi g. 55).] The
smallest female cells, which are presumably oogonia and
early primary oocytes (cell diameter 6 μm, nucleus diameter
5 μm), are situated on the periphery of the ovary in the germinal zone. They have a vesicular appearance with darkly
staining cytoplasm and relatively pale nuclei (Fig. 1.2A, B ).
Early and mid-stage developing oocytes (cell diameter
11–31 μm, nucleus diameter 7–12 μm) occur in the growth
zone. They are oval or polygonal with paler- or darkerstaining cytoplasm (Figs. 1.2C, D and 1.3A ). The cytoplasm
of the darker oocytes is relatively homogeneous, while in
lighter ones it is fi nely granular with pale inclusions
(Figs. 1.2C, D and 1.3A ).
Mature oocytes in the ovulatory zone (Fig. 1.3B, C ),
partly bulging into the zooidal cavity, attain 75–80 × 30–40 μm
diameter (nucleus 25 μm diameter). Thus, in the course of its
development in the ovary the volume of the female gamete
increases ca. 1,000-fold in this species.
The number of mature oocytes in the studied ovaries was
never greater than three. The presence in the ovary of similarsized oocytes indicates their synchronous development.
The cytoplasm of mature ovarian oocytes is differentiated
into central and peripheral zones (Fig. 1.3C ). The latter zone
is relatively pale, evenly stained and free from granules.
In contrast, numerous tiny granules (evidently yolk) are
concentrated in the central zone around the nucleus. As the
oocyte matures, the central zone enlarges and the granules
gradually occupy most of the cell. In passing it should be
noted that especially large intercellular spaces (putative subovarian zone) were seen in the basal part of those gonads that
contained growing oocytes with relatively few granules.
The nuclear envelope disappears before ovulation, with
mature oligolecithal (microlecithal) oocytes appearing in
the distal part of the zooidal coelom. The peripheral cytoplasmic zone in ovulated oocytes is free from yolk granules
and appears as a narrow, often very distinct dark rim
(Fig. 1.3D ).
The maximum total number of female cells of all ages
found in an ovary was 25, with up to nine ovulated oocytes
seen together in the zooidal cavity. Their size was
65–80 × 40–50 μm. In another case, there were 18 oocytes in
the ovary (including two late ones), and only one oocyte was
ovulated.
1.2.4 Reproductive Pattern II
in Cheilostomata
Pattern II of sexual reproduction is the commonest among
Cheilostomata. In general, it is characterized by formation in
the ovary of several (rarely numerous) macrolecithal oocytes
and their sequential (rarely near-simultaneous) maturation
and ovulation. In the course of oviposition, ovulated
oocyte(s) are transferred to a specialized brood chamber (in
some species, to the surface of the maternal zooid), where
they develop into endotrophic ciliated larvae. Embryos are
brooded in groups or one at a time. Fertilization is intraovarian and early. Karyogamy is delayed and occurs after oviposition. Oocytes develop in pairs with the nurse cell. There is
no extraembryonic nutrition.
It should be noted that the characteristics of this pattern are
somewhat different in brooding ctenostomes that have late
intraovarian fertilization and lack nurse cells (Marcus 1938a ;
Reed 1988 ; see also Sect. 3.4.4 ). Noteworthy, mesolecithal
oocytes also obviously develop in a few brooding cheilostomes with a primitive form of pattern II (see Chap. 3 ).
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
During the growth phase the oocyte increases in volume
ca. 6,000–8,000-fold. Towards the end of the vitellogenic
phase, oocytes lose contact with the subovarian space and
move into the centro-apical ovulatory zone where they are
partially exposed to the zooidal cavity and their oolemma
forms numerous microvilli. Hageman ( 1983 ) was not sure if
endocytosis occurred between their bases, however.
Following vitellogenesis and some time after the destruction
of the nuclear envelope oocytes ovulate, accumulating before
spawning in the coelom of the maternal zooid in groups of
20–30. Mature primary oocytes are shaped as a biconcave
elongated disc 85.8–101 μm in diameter.
Fertilization in malacostegans occurs during or shortly
after ovulation (Temkin 1994 , 1996 ). Sperm enter the coelom of fertile zooids via the intertentacular organ, through
which eggs are also spawned.
My data on oogenesis in Electra pilosa (Fig. 1.1A, B ) are
in generally good agreement with the results of Hageman
( 1983 ), as well as the earlier fi ndings of Calvet ( 1900 ) and
Bonnevie ( 1907 ). In this species the ovary is situated in the
proximal half of the autozooid, generally on the basal wall,
often adjoining one of the lateral walls (Figs. 1.2C, D
and 1.3B ). In some cases, however, only part of the gonad is
on the basal wall, the rest of it being suspended on funicular
cords in the zooidal cavity (Figs. 1.2A, B and 1.3A ). The
mature ovary often occupies a rather considerable area of the
basal-wall surface (Fig. 1.3B ).
The ovary consists of oogonia and oocytes surrounded by
the follicle cells (Figs. 1.2 and 1.3A ). The latter have an
irregular or fl attened oval shape and a size of 2.5–4.0 μm; the
staining of their cytoplasm and the nucleus is, in general,
similar to that of the epithelial cells of the zooid walls.
Intercellular spaces are mostly confi ned to the basal part of
the ovary; they are presumably parts of the subovarian space
revealed by Hageman ( 1983 ).
Oocytes of different size in the ovary are situated in three
main zones – germinal (peripheral), growth (subperipheral)
and ovulatory (central or subcentral), defi ned by their size
and appearance. [Between these zones are oocytes of intermediate size and the borders between them can barely be seen
by light microscopy (cf. Bonnevie 1907 , pl. 35, fi g. 55).] The
smallest female cells, which are presumably oogonia and
early primary oocytes (cell diameter 6 μm, nucleus diameter
5 μm), are situated on the periphery of the ovary in the germinal zone. They have a vesicular appearance with darkly
staining cytoplasm and relatively pale nuclei (Fig. 1.2A, B ).
Early and mid-stage developing oocytes (cell diameter
11–31 μm, nucleus diameter 7–12 μm) occur in the growth
zone. They are oval or polygonal with paler- or darkerstaining cytoplasm (Figs. 1.2C, D and 1.3A ). The cytoplasm
of the darker oocytes is relatively homogeneous, while in
lighter ones it is fi nely granular with pale inclusions
(Figs. 1.2C, D and 1.3A ).
Mature oocytes in the ovulatory zone (Fig. 1.3B, C ),
partly bulging into the zooidal cavity, attain 75–80 × 30–40 μm
diameter (nucleus 25 μm diameter). Thus, in the course of its
development in the ovary the volume of the female gamete
increases ca. 1,000-fold in this species.
The number of mature oocytes in the studied ovaries was
never greater than three. The presence in the ovary of similarsized oocytes indicates their synchronous development.
The cytoplasm of mature ovarian oocytes is differentiated
into central and peripheral zones (Fig. 1.3C ). The latter zone
is relatively pale, evenly stained and free from granules.
In contrast, numerous tiny granules (evidently yolk) are
concentrated in the central zone around the nucleus. As the
oocyte matures, the central zone enlarges and the granules
gradually occupy most of the cell. In passing it should be
noted that especially large intercellular spaces (putative subovarian zone) were seen in the basal part of those gonads that
contained growing oocytes with relatively few granules.
The nuclear envelope disappears before ovulation, with
mature oligolecithal (microlecithal) oocytes appearing in
the distal part of the zooidal coelom. The peripheral cytoplasmic zone in ovulated oocytes is free from yolk granules
and appears as a narrow, often very distinct dark rim
(Fig. 1.3D ).
The maximum total number of female cells of all ages
found in an ovary was 25, with up to nine ovulated oocytes
seen together in the zooidal cavity. Their size was
65–80 × 40–50 μm. In another case, there were 18 oocytes in
the ovary (including two late ones), and only one oocyte was
ovulated.
1.2.4 Reproductive Pattern II
in Cheilostomata
Pattern II of sexual reproduction is the commonest among
Cheilostomata. In general, it is characterized by formation in
the ovary of several (rarely numerous) macrolecithal oocytes
and their sequential (rarely near-simultaneous) maturation
and ovulation. In the course of oviposition, ovulated
oocyte(s) are transferred to a specialized brood chamber (in
some species, to the surface of the maternal zooid), where
they develop into endotrophic ciliated larvae. Embryos are
brooded in groups or one at a time. Fertilization is intraovarian and early. Karyogamy is delayed and occurs after oviposition. Oocytes develop in pairs with the nurse cell. There is
no extraembryonic nutrition.
It should be noted that the characteristics of this pattern are
somewhat different in brooding ctenostomes that have late
intraovarian fertilization and lack nurse cells (Marcus 1938a ;
Reed 1988 ; see also Sect. 3.4.4 ). Noteworthy, mesolecithal
oocytes also obviously develop in a few brooding cheilostomes with a primitive form of pattern II (see Chap. 3 ).
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis