124
IVAN QOODBODY
elles se confondent fac.ilemc*nt8 ; I(, rordon dorsal est constituk de cellules
indifferenci6ea. ”
Metcalf ( 189!)) cltiimed t l i t i t i t 1 scvcml species there is fusion between
the dorsal strand and the visccrd nerve, but the careful work of Maurice (1888), Hulls (1984), lirieii (1 927) and later Pedrle (1 938), Millar
(1963a), Aubert (1954) and Markman (1958) makes it clear that Metcalf
was mistaken and that the visceral nerve and the dorsal strand are
quite distinct structures. Fedele ( 1938) recognized that the dorsal
strand is not formed of ganglion cell8 and he considered that the cells
which form it are like neiiroblasts arid give rise to the nerve elements
which fltmk the wrd. He obsrrvcd in Cionw that the dorsal strand is
formed of xucbh undiflwcntii~ted cell8 which can detach and migrate into
the lateral tinsuex. Around the strand and ut some dixtance from it he
detected two layers of multipolur fusiform cells whose processes supply
a lateral plexus of fibres forming a sheath around the dorsal strand.
Scattered throughout the layer of multipolar cells he recognized pyriform motor cells like those found in the nerve ganglion. Fedele considered thiht fibres in the fibrous layer connect to the viscera, both
branchial sac and intestine. Pedele’s observations have since been
confirmed by Millar (1963a), Aubert (1954) and Markman (1958).
It is reasonable to conclude from these studies that the dorsal
strand is composed of undifferentinted cells surrounded by a fine nerve
plexus. In enterogoirous mcidian8 it forms an intimate link between
the neural glund uncl thc gonad, but in pleurogonous forms it ends
abruptly umongst thcl visceral. It is difficult to imagine what function
might be performed by a st>ructurc! of this sort. Huus (1924) was unable
to find the strand between the clonca m d the gonad and concluded that
it gave risc. to thc go1ioduc.t during development, but Aubert (1 954)
strongly refutes this on the grounds that in the adult it is possible to
demonstrate the entirety of the strand between gland and gonad, that
the strand and gonoduct coexist and that the gonoducts arise from the
gonad. Brien (1927) considered that, in blastogenesis of Aplidium
pallidum that portion of the strand enclosed in each bud gave rise to
the new neural complex and new dorsal strand as well as to gonadial
tissue although in a later work he omits reference to the gonads (Brien,
1948). Neither of these two conclusions (those of HUUS and Brien)
provides a11 adequate explanation of this enigmatical structure, nor
does it seem reasonable to assume as did Millar (1953a) that “ the dorsal
strand is functionless in the mature animal ”. If it was solely concerned
with blastogenesis we would not expect’ to find it in non-budding forms
like Ciona and Corella.
The intimate link betwcwi the neural gland and viscera Ruggests
IVAN QOODBODY
elles se confondent fac.ilemc*nt8 ; I(, rordon dorsal est constituk de cellules
indifferenci6ea. ”
Metcalf ( 189!)) cltiimed t l i t i t i t 1 scvcml species there is fusion between
the dorsal strand and the visccrd nerve, but the careful work of Maurice (1888), Hulls (1984), lirieii (1 927) and later Pedrle (1 938), Millar
(1963a), Aubert (1954) and Markman (1958) makes it clear that Metcalf
was mistaken and that the visceral nerve and the dorsal strand are
quite distinct structures. Fedele ( 1938) recognized that the dorsal
strand is not formed of ganglion cell8 and he considered that the cells
which form it are like neiiroblasts arid give rise to the nerve elements
which fltmk the wrd. He obsrrvcd in Cionw that the dorsal strand is
formed of xucbh undiflwcntii~ted cell8 which can detach and migrate into
the lateral tinsuex. Around the strand and ut some dixtance from it he
detected two layers of multipolur fusiform cells whose processes supply
a lateral plexus of fibres forming a sheath around the dorsal strand.
Scattered throughout the layer of multipolar cells he recognized pyriform motor cells like those found in the nerve ganglion. Fedele considered thiht fibres in the fibrous layer connect to the viscera, both
branchial sac and intestine. Pedele’s observations have since been
confirmed by Millar (1963a), Aubert (1954) and Markman (1958).
It is reasonable to conclude from these studies that the dorsal
strand is composed of undifferentinted cells surrounded by a fine nerve
plexus. In enterogoirous mcidian8 it forms an intimate link between
the neural glund uncl thc gonad, but in pleurogonous forms it ends
abruptly umongst thcl visceral. It is difficult to imagine what function
might be performed by a st>ructurc! of this sort. Huus (1924) was unable
to find the strand between the clonca m d the gonad and concluded that
it gave risc. to thc go1ioduc.t during development, but Aubert (1 954)
strongly refutes this on the grounds that in the adult it is possible to
demonstrate the entirety of the strand between gland and gonad, that
the strand and gonoduct coexist and that the gonoducts arise from the
gonad. Brien (1927) considered that, in blastogenesis of Aplidium
pallidum that portion of the strand enclosed in each bud gave rise to
the new neural complex and new dorsal strand as well as to gonadial
tissue although in a later work he omits reference to the gonads (Brien,
1948). Neither of these two conclusions (those of HUUS and Brien)
provides a11 adequate explanation of this enigmatical structure, nor
does it seem reasonable to assume as did Millar (1953a) that “ the dorsal
strand is functionless in the mature animal ”. If it was solely concerned
with blastogenesis we would not expect’ to find it in non-budding forms
like Ciona and Corella.
The intimate link betwcwi the neural gland and viscera Ruggests