298
from the nucleus outside the egg membrane, forming a special hyaline layer, the nucleus itself then degrading. Meiosis
begins [Bonnevie observed meiotic events and recorded a set
of 11 chromosomes in Electra pilosa ; later Temkin ( 1994 )
recorded a set of 12 chromosomes in the primary oocytes of
Membranipora membranacea ] while the mature egg is still
in the ovary, but does not continue in the zooid cavity after
ovulation. Further ovulated eggs increase in size, acquiring
variable shapes in E . pilosa . This increase is presumably not
growth per se , but rather enlargement caused by water entering the cytoplasm.
Like Silbermann ( 1906 ), Römer ( 1906 ) interpreted the
early germ cell, which he called an egg, as developing within
the epidermal layer of the cystid wall, not being connected
with the regenerating polypide bud in Alcyonidium sp. (as A .
mytili ). He suggested that the main reason for degeneration of
the new polypide is the development of the sex cells and
growth of the embryo that later fi lls the major part of the zooid.
Levinsen ( 1902 , 1909 ) described numerous variants of
ovicells and their development in different cheilostome taxa,
and introduced some basic terminology that is in common
use today. He stated that the egg should fi rst leave the zooid
via its opening before entering the brood chamber. Following
Jullien ( 1888b ), Levinsen ( 1909 , p. 66) suggested that in
some species, however, oviposition should occur underneath
the operculum with the help of the tentacle sheath, since
there is “an inner connection” between the maternal zooid
and ovicell that “form a common cavity”. The erroneous
statement concerning a “common cavity” was a consequence
of observations made on dried and cleaned material.
In their handbook, Korschelt and Heider ( 1910 ) briefl y
characterized ovicell structure based on the works of Nitsche
( 1869 ), Vigelius ( 1884a , b , 1886 ), Calvet ( 1900 ) and
Levinsen ( 1909 ). They pointed out the unsolved problem of
oviposition, mentioning the hypotheses of Vigelius (wrongly
ascribed to Calvet) and Levinsen. This question puzzled
many researchers at the time, but the observations of Pergens
( 1889 ) had been overlooked. In addition to the hypotheses
mentioned above, and in agreement with the idea of Nitsche
( 1869 ), Prouho ( 1892 ) presumed that there is a connection
between the ovicell incubatory chamber and the visceral coelom of the maternal zooid in Cheilostomata.
Three years after Korschelt and Heider’s ( 1910 ) textbook
and 24 years following Pergens’ ( 1889 ) paper, oviposition
was observed and described by Gerwerzhagen ( 1913 ) in
Bugula avicularia . He found that ovulation is caused by the
activity of the polypide, which presses upon and pushes the
ovary. According to him, fertilization occurs just after ovulation, since numerous sperm are present in the zooid cavity at
that moment. Oviposition is accompanied by violent exertions of the polypide, thanks to which the ovulated egg moves
into close proximity of the “Geburtsöffnung” [birth opening
or supraneural coelomopore]. Gerwerzhagen observed this
pore between the bases of two [dorsomedial] tentacles. Next,
the everted polypide takes up a special position close to the
ovicell opening, lowers its tentacles, and pushes the egg
toward the brood cavity. The contradiction between the relatively large size of the egg and the small diameter of the pore
is solved by the unusual plasticity of the egg, which stretches
out into a narrow cord. Gerwerzhagen supposed that this process could be facilitated by the sucking activity of the ovicell
itself via contraction of the muscles of the ooecial vesicle, but
he could not fi nd supportive evidence. Having accomplishing
oviposition, the polypide retracts, rests for some time, and
fi nally begins to feed again. If the polypide degenerates before
oviposition, the process takes place after polypide regeneration. Gerwerzhagen noted that he once observed the two-cell
stage of embryo development inside the maternal zooid. In
theory, it is possible that embryogenesis starts before oviposition when the polypide does not regenerate. In Membranipora
membranacea developing embryos were observed inside
zooids by Lutaud ( 1961 ).
Friedl ( 1925 ) made one of the fi rst seasonal observations
on the reproductive ecology of marine bryozoans, recording
the presence of yolky larvae within colonies of several species and cyphonautes larvae in the plankton. Some data on
the reproductive ecology of Bugula fl abellata were documented by Grave ( 1930 ).
Marcus ( 1926a ) investigated sexual reproduction in the
ctenostome Farrella repens and the cheilostome Electra
pilosa , and his observations supported the data of Van
Beneden ( 1844a ) and Bonnevie ( 1907 ). In particular, the testis was found on the funiculus and the ovary on the cystid
wall, and their development was both simultaneous and nonsimultaneous in the hermaphrodite zooids of Farrella . The
co-occurrence of mature sperm and eggs (up to ten in number) within the same zooids, inclined Marcus to accept selffertilization, but he also recorded sperm stuck to the tentacle
crown, suggesting that (1) this could be the result of simultaneous accidental release with liberation of eggs and (2) that
sperm should enter the zooidal cavity (again, through the
coelomopore) if cross-fertilization did in fact occur. In an
attempt to observe cross-fertilization in Electra , Marcus put
ovulated eggs and sperm in water together, but the spermatozoids died.
The ovary has been reported on the basal cystid wall in this
species, often in the proximal region of the zooid. Marcus
noted 10–20 mature ovarian oocytes and up to 17 ovulated
oocytes of various shapes in the zooid cavity. Spermatogenic
tissue develops in separate locations on the lateral and basal
walls also. Marcus more than once recorded the simultaneous
presence of male, female and hermaphrodite zooids in the
same colony, suggesting that all were hermaphrodite but at
different phases of their sexual cycle. He described egg liberation in detail, mentioning the strong deformation of the
eggs during their passage through the intertentacular organ in
Appendices
from the nucleus outside the egg membrane, forming a special hyaline layer, the nucleus itself then degrading. Meiosis
begins [Bonnevie observed meiotic events and recorded a set
of 11 chromosomes in Electra pilosa ; later Temkin ( 1994 )
recorded a set of 12 chromosomes in the primary oocytes of
Membranipora membranacea ] while the mature egg is still
in the ovary, but does not continue in the zooid cavity after
ovulation. Further ovulated eggs increase in size, acquiring
variable shapes in E . pilosa . This increase is presumably not
growth per se , but rather enlargement caused by water entering the cytoplasm.
Like Silbermann ( 1906 ), Römer ( 1906 ) interpreted the
early germ cell, which he called an egg, as developing within
the epidermal layer of the cystid wall, not being connected
with the regenerating polypide bud in Alcyonidium sp. (as A .
mytili ). He suggested that the main reason for degeneration of
the new polypide is the development of the sex cells and
growth of the embryo that later fi lls the major part of the zooid.
Levinsen ( 1902 , 1909 ) described numerous variants of
ovicells and their development in different cheilostome taxa,
and introduced some basic terminology that is in common
use today. He stated that the egg should fi rst leave the zooid
via its opening before entering the brood chamber. Following
Jullien ( 1888b ), Levinsen ( 1909 , p. 66) suggested that in
some species, however, oviposition should occur underneath
the operculum with the help of the tentacle sheath, since
there is “an inner connection” between the maternal zooid
and ovicell that “form a common cavity”. The erroneous
statement concerning a “common cavity” was a consequence
of observations made on dried and cleaned material.
In their handbook, Korschelt and Heider ( 1910 ) briefl y
characterized ovicell structure based on the works of Nitsche
( 1869 ), Vigelius ( 1884a , b , 1886 ), Calvet ( 1900 ) and
Levinsen ( 1909 ). They pointed out the unsolved problem of
oviposition, mentioning the hypotheses of Vigelius (wrongly
ascribed to Calvet) and Levinsen. This question puzzled
many researchers at the time, but the observations of Pergens
( 1889 ) had been overlooked. In addition to the hypotheses
mentioned above, and in agreement with the idea of Nitsche
( 1869 ), Prouho ( 1892 ) presumed that there is a connection
between the ovicell incubatory chamber and the visceral coelom of the maternal zooid in Cheilostomata.
Three years after Korschelt and Heider’s ( 1910 ) textbook
and 24 years following Pergens’ ( 1889 ) paper, oviposition
was observed and described by Gerwerzhagen ( 1913 ) in
Bugula avicularia . He found that ovulation is caused by the
activity of the polypide, which presses upon and pushes the
ovary. According to him, fertilization occurs just after ovulation, since numerous sperm are present in the zooid cavity at
that moment. Oviposition is accompanied by violent exertions of the polypide, thanks to which the ovulated egg moves
into close proximity of the “Geburtsöffnung” [birth opening
or supraneural coelomopore]. Gerwerzhagen observed this
pore between the bases of two [dorsomedial] tentacles. Next,
the everted polypide takes up a special position close to the
ovicell opening, lowers its tentacles, and pushes the egg
toward the brood cavity. The contradiction between the relatively large size of the egg and the small diameter of the pore
is solved by the unusual plasticity of the egg, which stretches
out into a narrow cord. Gerwerzhagen supposed that this process could be facilitated by the sucking activity of the ovicell
itself via contraction of the muscles of the ooecial vesicle, but
he could not fi nd supportive evidence. Having accomplishing
oviposition, the polypide retracts, rests for some time, and
fi nally begins to feed again. If the polypide degenerates before
oviposition, the process takes place after polypide regeneration. Gerwerzhagen noted that he once observed the two-cell
stage of embryo development inside the maternal zooid. In
theory, it is possible that embryogenesis starts before oviposition when the polypide does not regenerate. In Membranipora
membranacea developing embryos were observed inside
zooids by Lutaud ( 1961 ).
Friedl ( 1925 ) made one of the fi rst seasonal observations
on the reproductive ecology of marine bryozoans, recording
the presence of yolky larvae within colonies of several species and cyphonautes larvae in the plankton. Some data on
the reproductive ecology of Bugula fl abellata were documented by Grave ( 1930 ).
Marcus ( 1926a ) investigated sexual reproduction in the
ctenostome Farrella repens and the cheilostome Electra
pilosa , and his observations supported the data of Van
Beneden ( 1844a ) and Bonnevie ( 1907 ). In particular, the testis was found on the funiculus and the ovary on the cystid
wall, and their development was both simultaneous and nonsimultaneous in the hermaphrodite zooids of Farrella . The
co-occurrence of mature sperm and eggs (up to ten in number) within the same zooids, inclined Marcus to accept selffertilization, but he also recorded sperm stuck to the tentacle
crown, suggesting that (1) this could be the result of simultaneous accidental release with liberation of eggs and (2) that
sperm should enter the zooidal cavity (again, through the
coelomopore) if cross-fertilization did in fact occur. In an
attempt to observe cross-fertilization in Electra , Marcus put
ovulated eggs and sperm in water together, but the spermatozoids died.
The ovary has been reported on the basal cystid wall in this
species, often in the proximal region of the zooid. Marcus
noted 10–20 mature ovarian oocytes and up to 17 ovulated
oocytes of various shapes in the zooid cavity. Spermatogenic
tissue develops in separate locations on the lateral and basal
walls also. Marcus more than once recorded the simultaneous
presence of male, female and hermaphrodite zooids in the
same colony, suggesting that all were hermaphrodite but at
different phases of their sexual cycle. He described egg liberation in detail, mentioning the strong deformation of the
eggs during their passage through the intertentacular organ in
Appendices