2. THE NEUROHYPOPHYSIS
161
number of teleosts, and it is reasonable to suggest that they are concerned
either with sensory functions or with the conduction of neurosecretory
products to the ventricle. Despite such alternative routes, most workers
consider that neurosecretory materials leave the preoptic cells down the
main axons of the preoptico-hypophysial tract ( e.g., Bargmann and
Scharrer, 1951; Polenov, 1960).
The preoptic cells give rise to broad hillocks, which contain all the
structures of the main cell body, and in addition, long, thin, straight
canaliculi which have led Palay (Carassius auratus; 1960) to suggest
that the axons which they produce are, in fact, enlarged dendrites. These
fleshy, undulating, unmedullated axons form the preoptico-hypophysial
tract (Gadus morrhua; Lederis, 1962). They pass out of the nucleus in an
irregular and diffuse manner, often leaving in a lateral direction and then
bending ventrally before forming a discrete tract in the infundibular floor
(Dodd and Kerr, 1963; Platypoecilus maculatus, Oztan, 1963). In the
stickleback, Gasterosteus aculeatus, there is an extreme condition in
which the axons cross much of the infundibular floor as separate, lateral
tracts, and only come together to enter the pituitary itself (Dodd and
Kerr, 1963). The tract has been particularly clearly mapped in the eel,
Anguilla anguilla, by Leatherland et al., 1966 (Fig. 9 ) . Here, the pars
parvocellularis gives off four well-defined and ventrally located tracts,
which turn and run in a caudal direction for some distance, before they
are joined by a diffuse curtain of axons which descends from the large,
dorsally located cells of the pars magnocellularis; all these axons unite to
form a single tract which converges on the midline, above the optic
chiasma, and continues either past or through the nucleus lateralis tuberis
toward the pituitary (Stutinsky, 1953; Knowles and Vollrath, 1966a;
Leatherland et al., 1966).
The main preoptico-hypophysial tract of many teleosts has been
shown to carry rich accumulations of neurosecretory material, often in a
beaded form (e.g., Bargmann, 1953). Oztan ( 1963) has remarked on the
presence of Herring bodies in the tract of Platypoecilus maculatus, and
she has added the interesting observation that there is an increase in the
neurosecretion present in the tract of sterile fish produced as a back-cross
between this species and Xiphophorus helleri. Follenius (1963) has examined the neurosecretory droplets present in the tract of Salmo irideus
by electron microscopy, and he has shown that they consist of “packets”
of elementary vesicles, essentially similar to those present in the cell body,
but now grouped into discrete masses. There is no information concerning the mechanism of formation of these masses. Follenius has
shown that the elementary vesicles do not increase in size as they move
down the tract, and he has concluded that there is no extra synthesis by
161
number of teleosts, and it is reasonable to suggest that they are concerned
either with sensory functions or with the conduction of neurosecretory
products to the ventricle. Despite such alternative routes, most workers
consider that neurosecretory materials leave the preoptic cells down the
main axons of the preoptico-hypophysial tract ( e.g., Bargmann and
Scharrer, 1951; Polenov, 1960).
The preoptic cells give rise to broad hillocks, which contain all the
structures of the main cell body, and in addition, long, thin, straight
canaliculi which have led Palay (Carassius auratus; 1960) to suggest
that the axons which they produce are, in fact, enlarged dendrites. These
fleshy, undulating, unmedullated axons form the preoptico-hypophysial
tract (Gadus morrhua; Lederis, 1962). They pass out of the nucleus in an
irregular and diffuse manner, often leaving in a lateral direction and then
bending ventrally before forming a discrete tract in the infundibular floor
(Dodd and Kerr, 1963; Platypoecilus maculatus, Oztan, 1963). In the
stickleback, Gasterosteus aculeatus, there is an extreme condition in
which the axons cross much of the infundibular floor as separate, lateral
tracts, and only come together to enter the pituitary itself (Dodd and
Kerr, 1963). The tract has been particularly clearly mapped in the eel,
Anguilla anguilla, by Leatherland et al., 1966 (Fig. 9 ) . Here, the pars
parvocellularis gives off four well-defined and ventrally located tracts,
which turn and run in a caudal direction for some distance, before they
are joined by a diffuse curtain of axons which descends from the large,
dorsally located cells of the pars magnocellularis; all these axons unite to
form a single tract which converges on the midline, above the optic
chiasma, and continues either past or through the nucleus lateralis tuberis
toward the pituitary (Stutinsky, 1953; Knowles and Vollrath, 1966a;
Leatherland et al., 1966).
The main preoptico-hypophysial tract of many teleosts has been
shown to carry rich accumulations of neurosecretory material, often in a
beaded form (e.g., Bargmann, 1953). Oztan ( 1963) has remarked on the
presence of Herring bodies in the tract of Platypoecilus maculatus, and
she has added the interesting observation that there is an increase in the
neurosecretion present in the tract of sterile fish produced as a back-cross
between this species and Xiphophorus helleri. Follenius (1963) has examined the neurosecretory droplets present in the tract of Salmo irideus
by electron microscopy, and he has shown that they consist of “packets”
of elementary vesicles, essentially similar to those present in the cell body,
but now grouped into discrete masses. There is no information concerning the mechanism of formation of these masses. Follenius has
shown that the elementary vesicles do not increase in size as they move
down the tract, and he has concluded that there is no extra synthesis by