1. THE PITUITARY GLAND
87
other fishes, the neurohypophysis develops from the thin floor of the
infundibulum, dorsal to the pars intermedia. Above the pars distalis, the
infundibular floor remains thin, similar to that in primitive bony fishes
( Section 111). The neurohypophysis may be very small (e.g., Chimaera,
Meurling, 1967b), or together with the intermedia it may form two large
lateral lobes, as in ScyZZiorhinzrs ( Meurling, 1962; Knowles, 19sS). Neurosecretory fibers, AF + ve and A F - ve, myelinated and nonmyelinated,
penetrate downward to form a relatively thin nervous layer above the
intermedia, and also penetrate among the intermedia endocrine cells to
some extent. In older specimens of SquuZw, Meurling (1!362) described
diverticuli from the infundibular cavity lined with ependymal cells, which
penetrate with nervous processes into the pars intermedia, an arrangement similar to that in the eel and primitive actinopterygians (Sections
11, C and 111). The more dorsal region of the neurohypophysis is often
poor in stainable neurosecretory material, the greatest accumulation of
which is in the region immediately dorsal to the pars intermedia. Some
species display a distinct membrane between the neural tissue and the
intermedia cells, containing a network of blood capillaries; in these cases
there is little penetration of nerve fibers between the endocrine cells. In
other species, the membrane has largely disappeared, and the fibers may
penetrate between the cell cords either in broad bundles (e.g., S q l -
lwrhinus) or irregularly (e.g., Rub, Torpedo, and Pristiurms) ( Meurling,
1962,1963).
Knowles’ demonstration (1965) of Type A and Type B fibers in the
dogfish neurointermediate lobe has been discussed above. Meurling
(1963) also traced AF - ve fibers, presumably corresponding to Type B,
to terminations on pars intermedia cells. The origin of Type B or A F - ve
fibers is uncertain, although some at least probably come from the NLT
(Mellinger, 1962a). The A F + ve ( = Type A?) fibers have been traced
back to the NPO (Scharrer, 1952; Mellinger, 1962a; Mellinger et aZ.,
1962). The NPO Type A fibers have been traced into the neurointermediate lobe to terminations on pericapillary spaces, pituicytes, and
gland cells (Mellinger, 1962a; Knowles, 1965; Polenov and Belenky,
1965); and other Type A fibers pass to the ventral hypothalamus (anterior
infundibular floor) just above the dorsal lobe of the pars distalis, where
they become associated with a capillary network to form a median
eminence (see below). Type B fibers have also been traced to the median
eminence (Mellinger, 1962a; Mellinger et al., 1962; Chevins, 1968). The
existence of these two fiber types has been established in the hypothalamo-neurohypophysial tract and neurointermediate lobe of Raia
( Meurling, 1967c; Chevins, 1968), in addition to the dogfish, Scyllwrhinzrs
( Mellinger, 1963a; Knowles, 1965).
87
other fishes, the neurohypophysis develops from the thin floor of the
infundibulum, dorsal to the pars intermedia. Above the pars distalis, the
infundibular floor remains thin, similar to that in primitive bony fishes
( Section 111). The neurohypophysis may be very small (e.g., Chimaera,
Meurling, 1967b), or together with the intermedia it may form two large
lateral lobes, as in ScyZZiorhinzrs ( Meurling, 1962; Knowles, 19sS). Neurosecretory fibers, AF + ve and A F - ve, myelinated and nonmyelinated,
penetrate downward to form a relatively thin nervous layer above the
intermedia, and also penetrate among the intermedia endocrine cells to
some extent. In older specimens of SquuZw, Meurling (1!362) described
diverticuli from the infundibular cavity lined with ependymal cells, which
penetrate with nervous processes into the pars intermedia, an arrangement similar to that in the eel and primitive actinopterygians (Sections
11, C and 111). The more dorsal region of the neurohypophysis is often
poor in stainable neurosecretory material, the greatest accumulation of
which is in the region immediately dorsal to the pars intermedia. Some
species display a distinct membrane between the neural tissue and the
intermedia cells, containing a network of blood capillaries; in these cases
there is little penetration of nerve fibers between the endocrine cells. In
other species, the membrane has largely disappeared, and the fibers may
penetrate between the cell cords either in broad bundles (e.g., S q l -
lwrhinus) or irregularly (e.g., Rub, Torpedo, and Pristiurms) ( Meurling,
1962,1963).
Knowles’ demonstration (1965) of Type A and Type B fibers in the
dogfish neurointermediate lobe has been discussed above. Meurling
(1963) also traced AF - ve fibers, presumably corresponding to Type B,
to terminations on pars intermedia cells. The origin of Type B or A F - ve
fibers is uncertain, although some at least probably come from the NLT
(Mellinger, 1962a). The A F + ve ( = Type A?) fibers have been traced
back to the NPO (Scharrer, 1952; Mellinger, 1962a; Mellinger et aZ.,
1962). The NPO Type A fibers have been traced into the neurointermediate lobe to terminations on pericapillary spaces, pituicytes, and
gland cells (Mellinger, 1962a; Knowles, 1965; Polenov and Belenky,
1965); and other Type A fibers pass to the ventral hypothalamus (anterior
infundibular floor) just above the dorsal lobe of the pars distalis, where
they become associated with a capillary network to form a median
eminence (see below). Type B fibers have also been traced to the median
eminence (Mellinger, 1962a; Mellinger et al., 1962; Chevins, 1968). The
existence of these two fiber types has been established in the hypothalamo-neurohypophysial tract and neurointermediate lobe of Raia
( Meurling, 1967c; Chevins, 1968), in addition to the dogfish, Scyllwrhinzrs
( Mellinger, 1963a; Knowles, 1965).