3
who studied about 30 species of Gymnolaemata. Calvet
demonstrated that the position of the mature female gonad
(ovary) was usually stable within a species but might also vary,
both within a species and within a group. Criticizing previous
researchers and agreeing with Vigelius ( 1886 ), he indicated
that the source of gametes was “mesenchymal” tissue, showing
that early female cells are recognisable in the bud of the
forming zooid. According to him, as the polypide developed,
the ovary was displaced to the funiculus or the zooidal
wall. As for the origin of the cells of the ovary wall, Calvet
indicated that they were formed from mesothelial cells surrounding the developing ovary in some species, and from the
germ cell cluster (the central part of which differentiates into
oocytes and the peripheral part into the ovary wall) in others.
He attributed all bryozoans to ovi- and viviparous types,
describing the differences in their oogenesis and precisely
describing spermatogenesis as well. Calvet tended to think
that bryozoans had intrazooidal self-fertilization and stated
that he observed it being preceded by the formation of two
reduction bodies expelled from the mature egg. He thought
that polypide regeneration in the hermaphrodite zooid was
accompanied by the formation of new gonads, with the
oocytes of the previous polypide being fertilized by the
sperms of the next one.
Harmer ( 1902 ) was the fi rst to report extraembryonic
nutrition during brooding in bryozoans. He also described
the three main reproductive patterns of Cheilostomata,
having compared sizes and numbers of oocytes formed by
different bryozoan species and connected these characteristics with larval type and the presence or absence of placental
nutrition (Harmer 1926 ).
The taxonomic papers of Waters ( 1896a , b [1898] , 1900 ,
1904a , b , 1906 , 1907 , 1909 , 1910 , 1912 , 1913 , 1914 ,
1919(1921) ) contain valuable information on the internal
structure and sexual reproduction of bryozoans, since he
studied anatomical sections and tried to apply the data thus
obtained to bryozoan classifi cation. Besides the structure of
brooding organs, Waters described and/or illustrated the
position and structure of gonads, the number and size of
oocytes and the presence of placental analogues.
A series of publications by Retzius ( 1904 , 1905 , 1906 ,
1909 , 1910 ), presenting information on spermatogenesis and
sperm structure in four species of gymnolaemate bryozoans,
were the most thorough descriptions of their time. Bonnevie
( 1906 , 1907 ) studied the dynamics of the sexual colony
structure in relation to differences in the time of origin and
functioning of the gonads in hermaphrodite zooids as well as
ovary structure and oogenesis in two malacostegan cheilostomes. In her view, gonads could develop twice in the same
zooid during its lifetime, which was in accord with Calvet
( 1900 ). Bonnevie also noted that mature sperm formed clusters (spermatozeugmata), considering this as an adaptation
facilitating sperm movement in seawater. At the same time,
this observation contradicted her belief in self-fertilization in
bryozoans. She also thought that the polyspermy (fusion of
oocytes with several sperm) she observed was the consequence of sperm being arranged into clusters and that it was
common in the species studied. In passing, it can be noted
that Repiachoff ( 1876 ) was the fi rst to describe sperm aggregations in Bryozoa.
Bonnevie’s observations were supplemented by Marcus
( 1926a ), who also thought that the simultaneous presence of
different gametes in the cavity of the same zooid indicated
self-fertilization. Marcus described in detail egg release in a
broadcasting cheilostome, noting considerable deformation
of the eggs during their passage through the intertentacular
organ. He pointed that the formation of the fertilization envelope and the separation of polar bodies occurred soon after
egg release. Later, the reproductive features of non-brooding
cheilostomes were actively studied by Cook ( 1960 , 1962 ,
1964a ), Cook and Hayward ( 1966 ), Dudley ( 1973 ) and the
Mawataris ( 1975 ; Mawatari and Mawatari 1975 ).
The formation of germ cells from the mesenchyma of the
developing zooidal bud in a ctenostome bryozoan was
described by Pace ( 1906 ). Silbermann’s ( 1906 ) and Römer’s
( 1906 ) interpretation was that they were formed from the
ectoderm of the cystid wall. Faulkner ( 1933 ) was less
categorical; he studied a ctenostome in which germ cells
(“neoblasts”) fi rst appeared in the zone of actively dividing
cells of the developing polypide bud, in which the cell layers
of the cystid wall continued into the cell layers of the bud.
According to his interpretation, germ cells migrated between
the cell layers of the bilayered polypide bud and formed a
group between the epithelium of the developing stomach and
its mesothelial lining. In sterile zooids, totipotent “neoblasts”
took part in the formation of the gut whereas in future female
zooids they formed the ovary.
Having recorded the sequence of appearance of male and
female autozooidal polymorphs in Celleporella sp. colonies,
Marcus ( 1938a ) ascertained cross-fertilization in bryozoans.
The most important discoveries of this zoologist included
early intraovarian fertilization in several cheilostome bryozoans and the corroboration of the results of Harmer ( 1902 ,
1926 ) and Waters ( 1913 ) concerning the presence of extraembryonic nutrition in cheilostomes. In a subsequent study,
Marcus ( 1941а ) described for the fi rst time how oocytes
develop in pairs, one of them becoming a nurse cell.
An important study of the reproductive biology of three
cheilostome species was presented by Silén ( 1945 ), who
obtained the fi rst data on the duration of oogenesis and
brooding from colonies in aquaria. He also carefully
described oviposition. A subsequent seminal study proved
how cross-fertilization is achieved in bryozoans (Silén 1966 )
– he witnessed sperm being discharged via a pore in the tips
of the two dorso-medial tentacles in four malacostegan
species. Further observations have shown that, in all other
1.1 Brief Historical Overview of Studies on Gymnolaemate Gonado- and Gametogenesis and Fertilization
who studied about 30 species of Gymnolaemata. Calvet
demonstrated that the position of the mature female gonad
(ovary) was usually stable within a species but might also vary,
both within a species and within a group. Criticizing previous
researchers and agreeing with Vigelius ( 1886 ), he indicated
that the source of gametes was “mesenchymal” tissue, showing
that early female cells are recognisable in the bud of the
forming zooid. According to him, as the polypide developed,
the ovary was displaced to the funiculus or the zooidal
wall. As for the origin of the cells of the ovary wall, Calvet
indicated that they were formed from mesothelial cells surrounding the developing ovary in some species, and from the
germ cell cluster (the central part of which differentiates into
oocytes and the peripheral part into the ovary wall) in others.
He attributed all bryozoans to ovi- and viviparous types,
describing the differences in their oogenesis and precisely
describing spermatogenesis as well. Calvet tended to think
that bryozoans had intrazooidal self-fertilization and stated
that he observed it being preceded by the formation of two
reduction bodies expelled from the mature egg. He thought
that polypide regeneration in the hermaphrodite zooid was
accompanied by the formation of new gonads, with the
oocytes of the previous polypide being fertilized by the
sperms of the next one.
Harmer ( 1902 ) was the fi rst to report extraembryonic
nutrition during brooding in bryozoans. He also described
the three main reproductive patterns of Cheilostomata,
having compared sizes and numbers of oocytes formed by
different bryozoan species and connected these characteristics with larval type and the presence or absence of placental
nutrition (Harmer 1926 ).
The taxonomic papers of Waters ( 1896a , b [1898] , 1900 ,
1904a , b , 1906 , 1907 , 1909 , 1910 , 1912 , 1913 , 1914 ,
1919(1921) ) contain valuable information on the internal
structure and sexual reproduction of bryozoans, since he
studied anatomical sections and tried to apply the data thus
obtained to bryozoan classifi cation. Besides the structure of
brooding organs, Waters described and/or illustrated the
position and structure of gonads, the number and size of
oocytes and the presence of placental analogues.
A series of publications by Retzius ( 1904 , 1905 , 1906 ,
1909 , 1910 ), presenting information on spermatogenesis and
sperm structure in four species of gymnolaemate bryozoans,
were the most thorough descriptions of their time. Bonnevie
( 1906 , 1907 ) studied the dynamics of the sexual colony
structure in relation to differences in the time of origin and
functioning of the gonads in hermaphrodite zooids as well as
ovary structure and oogenesis in two malacostegan cheilostomes. In her view, gonads could develop twice in the same
zooid during its lifetime, which was in accord with Calvet
( 1900 ). Bonnevie also noted that mature sperm formed clusters (spermatozeugmata), considering this as an adaptation
facilitating sperm movement in seawater. At the same time,
this observation contradicted her belief in self-fertilization in
bryozoans. She also thought that the polyspermy (fusion of
oocytes with several sperm) she observed was the consequence of sperm being arranged into clusters and that it was
common in the species studied. In passing, it can be noted
that Repiachoff ( 1876 ) was the fi rst to describe sperm aggregations in Bryozoa.
Bonnevie’s observations were supplemented by Marcus
( 1926a ), who also thought that the simultaneous presence of
different gametes in the cavity of the same zooid indicated
self-fertilization. Marcus described in detail egg release in a
broadcasting cheilostome, noting considerable deformation
of the eggs during their passage through the intertentacular
organ. He pointed that the formation of the fertilization envelope and the separation of polar bodies occurred soon after
egg release. Later, the reproductive features of non-brooding
cheilostomes were actively studied by Cook ( 1960 , 1962 ,
1964a ), Cook and Hayward ( 1966 ), Dudley ( 1973 ) and the
Mawataris ( 1975 ; Mawatari and Mawatari 1975 ).
The formation of germ cells from the mesenchyma of the
developing zooidal bud in a ctenostome bryozoan was
described by Pace ( 1906 ). Silbermann’s ( 1906 ) and Römer’s
( 1906 ) interpretation was that they were formed from the
ectoderm of the cystid wall. Faulkner ( 1933 ) was less
categorical; he studied a ctenostome in which germ cells
(“neoblasts”) fi rst appeared in the zone of actively dividing
cells of the developing polypide bud, in which the cell layers
of the cystid wall continued into the cell layers of the bud.
According to his interpretation, germ cells migrated between
the cell layers of the bilayered polypide bud and formed a
group between the epithelium of the developing stomach and
its mesothelial lining. In sterile zooids, totipotent “neoblasts”
took part in the formation of the gut whereas in future female
zooids they formed the ovary.
Having recorded the sequence of appearance of male and
female autozooidal polymorphs in Celleporella sp. colonies,
Marcus ( 1938a ) ascertained cross-fertilization in bryozoans.
The most important discoveries of this zoologist included
early intraovarian fertilization in several cheilostome bryozoans and the corroboration of the results of Harmer ( 1902 ,
1926 ) and Waters ( 1913 ) concerning the presence of extraembryonic nutrition in cheilostomes. In a subsequent study,
Marcus ( 1941а ) described for the fi rst time how oocytes
develop in pairs, one of them becoming a nurse cell.
An important study of the reproductive biology of three
cheilostome species was presented by Silén ( 1945 ), who
obtained the fi rst data on the duration of oogenesis and
brooding from colonies in aquaria. He also carefully
described oviposition. A subsequent seminal study proved
how cross-fertilization is achieved in bryozoans (Silén 1966 )
– he witnessed sperm being discharged via a pore in the tips
of the two dorso-medial tentacles in four malacostegan
species. Further observations have shown that, in all other
1.1 Brief Historical Overview of Studies on Gymnolaemate Gonado- and Gametogenesis and Fertilization