304
show the embryo sac as an evagination of the vestibulum,
which is confi rmed by the following description of the
“hatching” larva, which “moves at fi rst out of the embryo sac
into the vestibule … through the broadened passage of the
sac” (p. 22). The polypide degenerates at the onset of brooding, during which time an embryo occupies most of the zooid
cavity. A new oocyte begins its growth in the ovary after larval release and polypide regeneration. Unfortunately, it is
impossible to state if there is extraembryonic nutrition during brooding in this species, since the size of the eggs and
embryos was not given, and scale bars and magnifi cations
are absent in the paper. However, there is some indirect evidence, such as the appearance of the brood-sac wall during
brooding and the size of the mature oocyte and larva relative
to the zooid size. Zimmer (personal communication in Reed
1991 ) recorded that the embryo grows during embryogenesis
in congeneric W . arcuata .
The splendid textbook of Hyman ( 1959 ) ranked among
the main sources consulted by two generations of zoologists.
She gave an extensive review on bryozoans (including their
sexual reproduction) and a very complete list of references.
However, much of the information concerning reproduction
was unfortunately imprecise, wrong or incorrectly interpreted, and this part of the monograph is now mainly of historical interest. The same can be said of the text-books by
Brien ( 1960 ) and Kaestner ( 1963 ). Interestingly, in discussing possible situations in which mature sperm might leave
the zooid cavity, Brien cited liberation through the “pore
génital” [supraneural coelomopore], the terminal pores of
the tentacles (mentioning Cori’s 1941 fi nding of sperm
inside the tentacle cavity) and during polypide recycling. In
the latter case, it was said that sperm could be released by the
regenerating polypide, being incorporated in the brown body
fi rst. This idea is similar to the suggestions of Borg ( 1947 , p.
375) who thought that polypide cycling is mainly “to empty
the genital cells”. Following previous authors, Brien discussed both (auto- and cross-) opportunities for fertilization
in Bryozoa.
A series of papers on ctenostome reproductive biology
were published during the 1950s and 1960s. It started with
the classical work of Braem ( 1951 ) (see above), who
described several different variants of brooding, and who
suggested an evolutionary trend towards better protection of
the embryos (reviewed in Ström 1977 ). Chrétien ( 1958 )
studied development of the ovary and described oogenesis in
detail in protandrous colonies (with gonochoristic zooids) of
Alcyonidium diaphanum (as A . gelatinosum ). She recognized “cellules femelles initiales” [oogonia] (p. 29) by the
presence of enlarged nuclei, lying between the epithelium
and mesothelium at the proximal end of the polypide bud, at
a stage when the latter comprises a hollow vesicle – the
future gut and rudiment of the lophophore. Early female
cells were said to be of “mesenchymatic origin”, and the
statement of Faulkner ( 1933 ) about their origin from the
region of polypide-bud proliferation was rejected. Following
mitotic divisions and differentiation, a group of 6–10 small
“young oocytes” is formed in which cell membranes were
deemed to appear a little later. A similar picture was recorded
by Pace ( 1906 ) in Flustrellidra hispida , and it is not clear if
the cell membranes were absent or simply not distinguishable by light microscopy. Chrétien identifi ed a series of
stages showing related events in the development of the
ovary and polypide cycle, following these events through the
autumn in the aquarium. “Cytoplasmatic growth” of the
oocytes began before complete differentiation of the polypide, and nutrition was presumed to be provided by specialized caecal cells – where the ovary contacts the gut, special
cells with tongue-like and papillate extensions protrude into
the caecal lumen. Peritoneal cells multiply, spreading over
the oocytes to form a follicle. Commencement of vitellogenesis is accompanied by further multiplication of the follicle
cells, forming a double layer. Such a double-layered follicle
was later recorded in Alcyonidium hirsutum by Owrid and
Ryland ( 1991 ). Judging from Chrétien’s description, fertilization should occur within the ovary, at a late stage of oocyte
development. On the other hand, polypide degeneration
begins before vitellogenesis starts, i.e. alien sperm should be
obtained by a zooid during much earlier stages of oocyte
development. Some 4–5 oocytes reach maturity, whereas the
rest are aborted. Mature oocytes occupy most of the zooidal
cavity, and their follicles consist of fl attened cells at this
stage. Chrétien carefully described vitellogenesis, starting
from the successive formation of several ribosomal aggregations (“caps”) by the nucleolus. Yolk granules are accumulated fi rst at the periphery, then throughout the ooplasm.
Additionally, in vitellogenic oocytes she demonstrated the
presence of large amounts of protein and polysaccharides as
well as numerous lipid droplets that are seen in later stages
using histochemistry.
Bobin and Prenant ( 1957 ) showed that polypide degeneration is connected with the maturation of the ovary in A .
gelatinosum . Grellet ( 1958 ) investigated testis structure and
spermatogenesis in A . diaphanum (as A . gelatinosum ), mentioning that spermatogenic tissue is associated with the
funiculus, which possibly supplies it with nutrients independently of the polypide. He also noted male germ cells in the
cystid peritoneum. Ranzoli ( 1962 ) studied the cytological
characters of the oocytes in Zoobotryon verticillatum .
Matricon ( 1963 ) found that the ovary develops in connection with the polypide and brooding of 4–6 embryos takes
place inside the incubation pouch, developing, after polypide degeneration, between the vestibule and degenerated
tentacle sheath in Alcyonidium polyoum . She suggested that
eggs enter the brood pouch through the newly developed
ciliary funnel leading to the supraneural pore. In another
paper, Matricon ( 1960 ) recorded testes developing on the
lateral and basal cystid walls in this species. Banta ( 1968 )
described larval brooding in the tentacle sheath in
Appendices
show the embryo sac as an evagination of the vestibulum,
which is confi rmed by the following description of the
“hatching” larva, which “moves at fi rst out of the embryo sac
into the vestibule … through the broadened passage of the
sac” (p. 22). The polypide degenerates at the onset of brooding, during which time an embryo occupies most of the zooid
cavity. A new oocyte begins its growth in the ovary after larval release and polypide regeneration. Unfortunately, it is
impossible to state if there is extraembryonic nutrition during brooding in this species, since the size of the eggs and
embryos was not given, and scale bars and magnifi cations
are absent in the paper. However, there is some indirect evidence, such as the appearance of the brood-sac wall during
brooding and the size of the mature oocyte and larva relative
to the zooid size. Zimmer (personal communication in Reed
1991 ) recorded that the embryo grows during embryogenesis
in congeneric W . arcuata .
The splendid textbook of Hyman ( 1959 ) ranked among
the main sources consulted by two generations of zoologists.
She gave an extensive review on bryozoans (including their
sexual reproduction) and a very complete list of references.
However, much of the information concerning reproduction
was unfortunately imprecise, wrong or incorrectly interpreted, and this part of the monograph is now mainly of historical interest. The same can be said of the text-books by
Brien ( 1960 ) and Kaestner ( 1963 ). Interestingly, in discussing possible situations in which mature sperm might leave
the zooid cavity, Brien cited liberation through the “pore
génital” [supraneural coelomopore], the terminal pores of
the tentacles (mentioning Cori’s 1941 fi nding of sperm
inside the tentacle cavity) and during polypide recycling. In
the latter case, it was said that sperm could be released by the
regenerating polypide, being incorporated in the brown body
fi rst. This idea is similar to the suggestions of Borg ( 1947 , p.
375) who thought that polypide cycling is mainly “to empty
the genital cells”. Following previous authors, Brien discussed both (auto- and cross-) opportunities for fertilization
in Bryozoa.
A series of papers on ctenostome reproductive biology
were published during the 1950s and 1960s. It started with
the classical work of Braem ( 1951 ) (see above), who
described several different variants of brooding, and who
suggested an evolutionary trend towards better protection of
the embryos (reviewed in Ström 1977 ). Chrétien ( 1958 )
studied development of the ovary and described oogenesis in
detail in protandrous colonies (with gonochoristic zooids) of
Alcyonidium diaphanum (as A . gelatinosum ). She recognized “cellules femelles initiales” [oogonia] (p. 29) by the
presence of enlarged nuclei, lying between the epithelium
and mesothelium at the proximal end of the polypide bud, at
a stage when the latter comprises a hollow vesicle – the
future gut and rudiment of the lophophore. Early female
cells were said to be of “mesenchymatic origin”, and the
statement of Faulkner ( 1933 ) about their origin from the
region of polypide-bud proliferation was rejected. Following
mitotic divisions and differentiation, a group of 6–10 small
“young oocytes” is formed in which cell membranes were
deemed to appear a little later. A similar picture was recorded
by Pace ( 1906 ) in Flustrellidra hispida , and it is not clear if
the cell membranes were absent or simply not distinguishable by light microscopy. Chrétien identifi ed a series of
stages showing related events in the development of the
ovary and polypide cycle, following these events through the
autumn in the aquarium. “Cytoplasmatic growth” of the
oocytes began before complete differentiation of the polypide, and nutrition was presumed to be provided by specialized caecal cells – where the ovary contacts the gut, special
cells with tongue-like and papillate extensions protrude into
the caecal lumen. Peritoneal cells multiply, spreading over
the oocytes to form a follicle. Commencement of vitellogenesis is accompanied by further multiplication of the follicle
cells, forming a double layer. Such a double-layered follicle
was later recorded in Alcyonidium hirsutum by Owrid and
Ryland ( 1991 ). Judging from Chrétien’s description, fertilization should occur within the ovary, at a late stage of oocyte
development. On the other hand, polypide degeneration
begins before vitellogenesis starts, i.e. alien sperm should be
obtained by a zooid during much earlier stages of oocyte
development. Some 4–5 oocytes reach maturity, whereas the
rest are aborted. Mature oocytes occupy most of the zooidal
cavity, and their follicles consist of fl attened cells at this
stage. Chrétien carefully described vitellogenesis, starting
from the successive formation of several ribosomal aggregations (“caps”) by the nucleolus. Yolk granules are accumulated fi rst at the periphery, then throughout the ooplasm.
Additionally, in vitellogenic oocytes she demonstrated the
presence of large amounts of protein and polysaccharides as
well as numerous lipid droplets that are seen in later stages
using histochemistry.
Bobin and Prenant ( 1957 ) showed that polypide degeneration is connected with the maturation of the ovary in A .
gelatinosum . Grellet ( 1958 ) investigated testis structure and
spermatogenesis in A . diaphanum (as A . gelatinosum ), mentioning that spermatogenic tissue is associated with the
funiculus, which possibly supplies it with nutrients independently of the polypide. He also noted male germ cells in the
cystid peritoneum. Ranzoli ( 1962 ) studied the cytological
characters of the oocytes in Zoobotryon verticillatum .
Matricon ( 1963 ) found that the ovary develops in connection with the polypide and brooding of 4–6 embryos takes
place inside the incubation pouch, developing, after polypide degeneration, between the vestibule and degenerated
tentacle sheath in Alcyonidium polyoum . She suggested that
eggs enter the brood pouch through the newly developed
ciliary funnel leading to the supraneural pore. In another
paper, Matricon ( 1960 ) recorded testes developing on the
lateral and basal cystid walls in this species. Banta ( 1968 )
described larval brooding in the tentacle sheath in
Appendices