130
A. M. PERKS
lamina the axons converge and form a discrete tract close to the midline
( Squalus acanthias, Meurling, 1967a). Here the tract shows densely
packed neurosecretory granules and Herring bodies in many-but not
all-of its fibers ( Scyliorhinus stellarb, Scharrer, 1952; Dasyatis mrinus,
Bargmann, 1955; Scyliorhinus caniculus, Perks, 1959; Mellinger et at.,
1962; etc. ) . In this region, portal capillaries penetrate the tract (Scyliorhinus caniculus, S . stellaris, Raia asterias, R. punctata, R. unduluta,
Legait, 1957; Scyliorhinus caniculus, Bugnon and Lenys, 1961, 1962) ,
and a median eminence is formed (Scyliorhinus caniculus, Squalus acanthias, Raia batb, R. radiata, Meurhg, 1960; Scyliorhinus caniculus,
Mellinger, 1960). In Scyliorhinus caniculus, neurosecretory material has
been seen close to the blood vessels which invade the tract, and light and
electron microscopy has shown that isolated neurosecretory axons, surrounded by a coat of glial cells, leave the main tract and form terminals
on the penetrating capillaries: These terminals contain a central mass of
synaptic vesicles and abundant neurosecretory granules, and it is considered probable that neurosecretory materials may pass from them into
the capillaries ( Scyliorhinus caniculus, Mellinger, 1960; Mellinger et al.,
1962; many species, Meurling, 1967a). The capillaries drain into a complex of portal vessels which are the sole blood supply of the rostra1 lobe
of the adenohypophysis (pars rostralis and pars medialis of Meurling,
1967a) ; this arrangement strongly suggests a mechanism for control of
the adenohypophysial cells. However, in Scylwrhinus caniculus, Pristiurus
melanostomus, and various species of Raia, portal vessels also supply the
neurointermediate lobe of the pituitary ( Meurling, 1967a), a situation
strangely similar to that of the hagfish.
The majority of the axons of the preoptico-hypophysial tract do not
terminate in the median eminence but pass through the thin hypophysial
stem toward the pituitary. In Raia radiata, this thin stem contains a wide
band of nerve fibers, which may fall into as many as six different types,
although some may be intermediate forms or variations along a single
fiber (Meurling, 196%). The axons are partly localized into particular
groups. The lateral region contains mainly nonneurosecretory axons,
which are of largely unknown significance. The medial region of the stalk
contains predominantly Gomori-positive neurosecretory fibers. The electron microscope has shown that these contain large, electron-dense neurosecretory vesicles, 20003000 A in diameter, probably comparable to the
Type A elementary vesicles described in the neurointermediate lobe of
Scyliorhinus caniculus (Knowles, 1965). In addition, there are axons
which contain smaller, electron-dense vesicles, 1OOO-1300 A in diameter,
which have a distinct external membrane, and may be comparable to the
Type B elementary vesicles of Scyliorhinus caniculus ( Knowles, 1965). In
A. M. PERKS
lamina the axons converge and form a discrete tract close to the midline
( Squalus acanthias, Meurling, 1967a). Here the tract shows densely
packed neurosecretory granules and Herring bodies in many-but not
all-of its fibers ( Scyliorhinus stellarb, Scharrer, 1952; Dasyatis mrinus,
Bargmann, 1955; Scyliorhinus caniculus, Perks, 1959; Mellinger et at.,
1962; etc. ) . In this region, portal capillaries penetrate the tract (Scyliorhinus caniculus, S . stellaris, Raia asterias, R. punctata, R. unduluta,
Legait, 1957; Scyliorhinus caniculus, Bugnon and Lenys, 1961, 1962) ,
and a median eminence is formed (Scyliorhinus caniculus, Squalus acanthias, Raia batb, R. radiata, Meurhg, 1960; Scyliorhinus caniculus,
Mellinger, 1960). In Scyliorhinus caniculus, neurosecretory material has
been seen close to the blood vessels which invade the tract, and light and
electron microscopy has shown that isolated neurosecretory axons, surrounded by a coat of glial cells, leave the main tract and form terminals
on the penetrating capillaries: These terminals contain a central mass of
synaptic vesicles and abundant neurosecretory granules, and it is considered probable that neurosecretory materials may pass from them into
the capillaries ( Scyliorhinus caniculus, Mellinger, 1960; Mellinger et al.,
1962; many species, Meurling, 1967a). The capillaries drain into a complex of portal vessels which are the sole blood supply of the rostra1 lobe
of the adenohypophysis (pars rostralis and pars medialis of Meurling,
1967a) ; this arrangement strongly suggests a mechanism for control of
the adenohypophysial cells. However, in Scylwrhinus caniculus, Pristiurus
melanostomus, and various species of Raia, portal vessels also supply the
neurointermediate lobe of the pituitary ( Meurling, 1967a), a situation
strangely similar to that of the hagfish.
The majority of the axons of the preoptico-hypophysial tract do not
terminate in the median eminence but pass through the thin hypophysial
stem toward the pituitary. In Raia radiata, this thin stem contains a wide
band of nerve fibers, which may fall into as many as six different types,
although some may be intermediate forms or variations along a single
fiber (Meurling, 196%). The axons are partly localized into particular
groups. The lateral region contains mainly nonneurosecretory axons,
which are of largely unknown significance. The medial region of the stalk
contains predominantly Gomori-positive neurosecretory fibers. The electron microscope has shown that these contain large, electron-dense neurosecretory vesicles, 20003000 A in diameter, probably comparable to the
Type A elementary vesicles described in the neurointermediate lobe of
Scyliorhinus caniculus (Knowles, 1965). In addition, there are axons
which contain smaller, electron-dense vesicles, 1OOO-1300 A in diameter,
which have a distinct external membrane, and may be comparable to the
Type B elementary vesicles of Scyliorhinus caniculus ( Knowles, 1965). In