2. THE NEUROHYPOPHYSIS
147
teans) is a tract which divides away from the main trunk in the
infundibular floor; it forms a ventral branch, which is less well defined
than the main tract. This branch becomes closely associated with a plexus
of blood vessels, which may penetrate relatively deeply into the ventral
surface of the infundibulum (species as above, Kerr, 1 W ) . Neurosecretory granules have been seen within the palisade layer which borders
these capillaries (Polypterus sp., Wingstrand, 1966). The capillaries
appear to connect to the adenohypophysis and perhaps with the neurointermediate tissue; this strongly suggests that a median eminencepituitary portal system is present in the brachiopterygians. Further, this
indicates that a pituitary portal system may have been a primitive feature
of the bony fish (Wingstrand, 1966; Kerr, 1968).
The major trunk of the preoptico-hypophysial tract continues past the
region of the median eminence and is deeply buried in the floor of the
tubular infundibulum. It carries neurosecretory granules throughout its
length, and these are particularly concentrated toward its ventral surface.
Although a few fibers terminate in the s a c a s vasculosus, most penetrate
into the caudal infundibular floor, which is thin in species of Calamoichthys (Fig. 6), but may be thickened in species of Polypterus (Kerr,
1988; Dodd and Kerr, 1963; Charipper, 1937). The infundibular floor
produces short, tubelike projections, which arise irregularly on both sides
of the midline in a pattern which Kerr (1968) has considered to be
primitive. The tubes, which penetrate into the underlying pars intermedia,
soon consolidate into solid rods, which retain a central core of ependymal
cells. They may contain large ganglion cells (Calamoichthys calubaricus;
Bickford, 1895). The axons of the preoptico-hypophysial tract enter these
projections, which are rich in neurosecretory material and constitute the
pars nervosa. Herring bodies have been seen lining the outer surface of
the neural processes, in close association with intermedia cells. (Polypt e r n senegalis, Dodd and Kerr, 1963; Wingstrand, 1966; Kerr, 1968;
Calamoichthys calubaricus, Polyptems bichir, Kerr, 1968). The neural
processes intermingle with the pars intermedia in a manner which may
be more complex and intimate than that seen in the teleost pituitary
( Calumoichthys calabaricus, Bickford, 1895; Polyptews sp., de Beer, 1926;
as above, Kerr, 1968). However, it is probable that there is little penetration into other regions of the adenohypophysis. Nevertheless, the pars
intermedia, which is relatively small in some species (Calamoichthys sp.,
Dodd and Kerr, 1963), does not completely invest the neural processes in
all specimens of Polypterus (Charipper, 1937), and Kerr ( 1968) has
noted one exceptional invasion of neural tissue into the anterior regions
of the adenohypophysis of this species. This intrusion could foreshadow
an evolutionary trend toward the extensive neural penetration seen in the
147
teans) is a tract which divides away from the main trunk in the
infundibular floor; it forms a ventral branch, which is less well defined
than the main tract. This branch becomes closely associated with a plexus
of blood vessels, which may penetrate relatively deeply into the ventral
surface of the infundibulum (species as above, Kerr, 1 W ) . Neurosecretory granules have been seen within the palisade layer which borders
these capillaries (Polypterus sp., Wingstrand, 1966). The capillaries
appear to connect to the adenohypophysis and perhaps with the neurointermediate tissue; this strongly suggests that a median eminencepituitary portal system is present in the brachiopterygians. Further, this
indicates that a pituitary portal system may have been a primitive feature
of the bony fish (Wingstrand, 1966; Kerr, 1968).
The major trunk of the preoptico-hypophysial tract continues past the
region of the median eminence and is deeply buried in the floor of the
tubular infundibulum. It carries neurosecretory granules throughout its
length, and these are particularly concentrated toward its ventral surface.
Although a few fibers terminate in the s a c a s vasculosus, most penetrate
into the caudal infundibular floor, which is thin in species of Calamoichthys (Fig. 6), but may be thickened in species of Polypterus (Kerr,
1988; Dodd and Kerr, 1963; Charipper, 1937). The infundibular floor
produces short, tubelike projections, which arise irregularly on both sides
of the midline in a pattern which Kerr (1968) has considered to be
primitive. The tubes, which penetrate into the underlying pars intermedia,
soon consolidate into solid rods, which retain a central core of ependymal
cells. They may contain large ganglion cells (Calamoichthys calubaricus;
Bickford, 1895). The axons of the preoptico-hypophysial tract enter these
projections, which are rich in neurosecretory material and constitute the
pars nervosa. Herring bodies have been seen lining the outer surface of
the neural processes, in close association with intermedia cells. (Polypt e r n senegalis, Dodd and Kerr, 1963; Wingstrand, 1966; Kerr, 1968;
Calamoichthys calubaricus, Polyptems bichir, Kerr, 1968). The neural
processes intermingle with the pars intermedia in a manner which may
be more complex and intimate than that seen in the teleost pituitary
( Calumoichthys calabaricus, Bickford, 1895; Polyptews sp., de Beer, 1926;
as above, Kerr, 1968). However, it is probable that there is little penetration into other regions of the adenohypophysis. Nevertheless, the pars
intermedia, which is relatively small in some species (Calamoichthys sp.,
Dodd and Kerr, 1963), does not completely invest the neural processes in
all specimens of Polypterus (Charipper, 1937), and Kerr ( 1968) has
noted one exceptional invasion of neural tissue into the anterior regions
of the adenohypophysis of this species. This intrusion could foreshadow
an evolutionary trend toward the extensive neural penetration seen in the