THE t'I
muscle contractmit which mimicv thc act,ivity of oxytocin. Because of
the small size of the neural gltind i t is not possible to be certain of
working with it alone and most iissay work is carried out on the complex
of neural gland and ganglion. I n consequence we cannot be sure
whether the smooth muscle contractant (8MC) resides in the tissues of
the neural gland or in the neurosecretory cells of the ganglion (Dawson
and Hisaw, 1964 ; Lane, 1968). Furthermore SMC activity is also found,
though less strongly, in other tissues of the animal m d it may therefore
be a generally distributed substruice in a~cidian tissues or it may be a
hormone mainttlined in circulation in the blood. It is not clear what
function such a Substance might serve in the living ascidian, particularly
in view of the fact that all ascidian musclo is non-striated. However, i t
is not beyond the bounds of possibility that it might be specific for one
spocialized musde and maintain tonus in the sphincter guarding the
genital opening and preventing premature release of sperm and eggs
(see below). This would necessitate a mechanism for the inhibition of
secretion and relaxation of the muscle at spawning.
2. Gonadotrophins
In spite of the fact that the embryological evidence demonstrates
that there is no stomodeal component in the formation of the neural
gland and that therefore it can have no morphological homology with
the vertebrate anterior pituitary a great many workers have sought
ovidence for the presence of goiiadotrophins in the neural complex.
The resulting information productis a confusing and conflicting array of
data. Some of the confusion probably arise8 from a misunderstanding
of the ovarian cycle and spawning behaviour of ascidians. €'6r&s (1964)
has drawn attention to the fact that in sexually mature ascidians, and
he refers specifically to Ciom, the ovarian cycle can be divided into two
parts, an intercycle and a period of germinal growth. During the intercyole the germinal epithelium is reduced and a somatic epithelium
expends; oogonia are rare and the remaining oocytes break down.
When the period of germinal growth commences there is a rapid
development of oogonia and production of oocytes which are discharged
into the lumen of the gonadial cavity and complete their maturation
there or in the oviduct. Egg production then becomes a continuous
process and the eggs me stored up in the oviduct which prior to spawning becomes greatly distended. It seems then that the liberation of
gametes at spawning is not a function of increased gonadial activity but
must be the result, of relaxation of the sphincter guarding the gonopore
together with contraction of the mantle muscle to increase the hgdrostatic pressure in the mantle cavity and thereby squeeze out thc
5
A M B.-I'L
muscle contractmit which mimicv thc act,ivity of oxytocin. Because of
the small size of the neural gltind i t is not possible to be certain of
working with it alone and most iissay work is carried out on the complex
of neural gland and ganglion. I n consequence we cannot be sure
whether the smooth muscle contractant (8MC) resides in the tissues of
the neural gland or in the neurosecretory cells of the ganglion (Dawson
and Hisaw, 1964 ; Lane, 1968). Furthermore SMC activity is also found,
though less strongly, in other tissues of the animal m d it may therefore
be a generally distributed substruice in a~cidian tissues or it may be a
hormone mainttlined in circulation in the blood. It is not clear what
function such a Substance might serve in the living ascidian, particularly
in view of the fact that all ascidian musclo is non-striated. However, i t
is not beyond the bounds of possibility that it might be specific for one
spocialized musde and maintain tonus in the sphincter guarding the
genital opening and preventing premature release of sperm and eggs
(see below). This would necessitate a mechanism for the inhibition of
secretion and relaxation of the muscle at spawning.
2. Gonadotrophins
In spite of the fact that the embryological evidence demonstrates
that there is no stomodeal component in the formation of the neural
gland and that therefore it can have no morphological homology with
the vertebrate anterior pituitary a great many workers have sought
ovidence for the presence of goiiadotrophins in the neural complex.
The resulting information productis a confusing and conflicting array of
data. Some of the confusion probably arise8 from a misunderstanding
of the ovarian cycle and spawning behaviour of ascidians. €'6r&s (1964)
has drawn attention to the fact that in sexually mature ascidians, and
he refers specifically to Ciom, the ovarian cycle can be divided into two
parts, an intercycle and a period of germinal growth. During the intercyole the germinal epithelium is reduced and a somatic epithelium
expends; oogonia are rare and the remaining oocytes break down.
When the period of germinal growth commences there is a rapid
development of oogonia and production of oocytes which are discharged
into the lumen of the gonadial cavity and complete their maturation
there or in the oviduct. Egg production then becomes a continuous
process and the eggs me stored up in the oviduct which prior to spawning becomes greatly distended. It seems then that the liberation of
gametes at spawning is not a function of increased gonadial activity but
must be the result, of relaxation of the sphincter guarding the gonopore
together with contraction of the mantle muscle to increase the hgdrostatic pressure in the mantle cavity and thereby squeeze out thc
5
A M B.-I'L