2. THE NEUROHYPOPHYSIS
131
some cases, these latter fibers form terminal swellings within the stalk.
It has been suggested that they may affect the nearby pars medialis of the
adenohypophysis, or perhaps liberate neurovascular transmitters into the
nearby portal vessels, which could mediate an influence on the pars
intermedia ( Meurling, 196%). As the neurosecretory fibers continue in
a caudal direction, glial elements and secretory cells become less frequent,
and it is probable that the secretory cells are lost before the axons of the
preoptico-hypophysial tract enter the neurointermediate lobe of the pituitary. Here, axons bearing granules and Herring bodies, and some devoid
of secretion (Dasyatis marinus, Bargmann, 1955) pass below the single
or multiple layers of ependymal cells which line the ventricle (Squalus
acanthias; Meurling, 1962). The overlying ependymal cells send down
fibers which divide the tract into bundles (Squalus acanthim; Meurling,
1962). In Squalus acanthias and Scyliorhinus caniculus some ependymal
cells which contain vacuoles are found enclosed within the tract itself
(“parenchymatous pituicytes,” van der Kamer and Verhagen, 1955;
Meurling, 19f32). In some species the neurosecretory axons terminate in
a fairly well defined pars nervosa, but in others the terminations penetrate diffusely throughout the pars intermedia of the pituitary. The welldefined pars nervosa of Squalus acanthias may be a primitive feature
( Meurling, 1962). In this species the neurosecretory axons carry variable
numbers of granules of different sizes, and they are sometimes in the form
of beads (Meurling, 1962). The axons are confined mainly within a
lamina of variable thickness which covers the dorsal surface of the pars
intermedia. This lamina sends down solid and tubelike processes into the
intermedia tissue, and these projections, together with the dorsal neural
covering, constitute the major portion of the pars nervosa. In the primitive sharks, Heranchus, Heptanchus, and Chlumydoselachus, similar rodlike and tubelike processes are well developed (Stendell, 1914; Norris,
1941 ) . Within the pars nervosa of Squalus acanthias most axons terminate
in an outer layer of ependymal fibers which follows the boundary with
the pars intermedia. This layer contains a dense accumulation of neurosecretion. It contacts an external reticular membrane, which encompasses
a network of capillaries. This neural lobe plexus forms the boundary between the pars nervosa and the pars intermedia. It drains into the sinuses
of the intermedia tissue and could form a neurovascular link between the
two areas of the pituitary (Meurling, 1962, 1967a). This is reminiscent
of the situation in the lampreys. However, neurosecretory axons do enter
the pars intermedia of Squalus acanthias to a small extent (Fig. 4;
Meurling, 1962). Sometimes isolated axons penetrate the boundary membrane and take an irregular course between the intermedia cells. More
often, and particularly in the neural processes, groups of neurosecretory
131
some cases, these latter fibers form terminal swellings within the stalk.
It has been suggested that they may affect the nearby pars medialis of the
adenohypophysis, or perhaps liberate neurovascular transmitters into the
nearby portal vessels, which could mediate an influence on the pars
intermedia ( Meurling, 196%). As the neurosecretory fibers continue in
a caudal direction, glial elements and secretory cells become less frequent,
and it is probable that the secretory cells are lost before the axons of the
preoptico-hypophysial tract enter the neurointermediate lobe of the pituitary. Here, axons bearing granules and Herring bodies, and some devoid
of secretion (Dasyatis marinus, Bargmann, 1955) pass below the single
or multiple layers of ependymal cells which line the ventricle (Squalus
acanthias; Meurling, 1962). The overlying ependymal cells send down
fibers which divide the tract into bundles (Squalus acanthim; Meurling,
1962). In Squalus acanthias and Scyliorhinus caniculus some ependymal
cells which contain vacuoles are found enclosed within the tract itself
(“parenchymatous pituicytes,” van der Kamer and Verhagen, 1955;
Meurling, 19f32). In some species the neurosecretory axons terminate in
a fairly well defined pars nervosa, but in others the terminations penetrate diffusely throughout the pars intermedia of the pituitary. The welldefined pars nervosa of Squalus acanthias may be a primitive feature
( Meurling, 1962). In this species the neurosecretory axons carry variable
numbers of granules of different sizes, and they are sometimes in the form
of beads (Meurling, 1962). The axons are confined mainly within a
lamina of variable thickness which covers the dorsal surface of the pars
intermedia. This lamina sends down solid and tubelike processes into the
intermedia tissue, and these projections, together with the dorsal neural
covering, constitute the major portion of the pars nervosa. In the primitive sharks, Heranchus, Heptanchus, and Chlumydoselachus, similar rodlike and tubelike processes are well developed (Stendell, 1914; Norris,
1941 ) . Within the pars nervosa of Squalus acanthias most axons terminate
in an outer layer of ependymal fibers which follows the boundary with
the pars intermedia. This layer contains a dense accumulation of neurosecretion. It contacts an external reticular membrane, which encompasses
a network of capillaries. This neural lobe plexus forms the boundary between the pars nervosa and the pars intermedia. It drains into the sinuses
of the intermedia tissue and could form a neurovascular link between the
two areas of the pituitary (Meurling, 1962, 1967a). This is reminiscent
of the situation in the lampreys. However, neurosecretory axons do enter
the pars intermedia of Squalus acanthias to a small extent (Fig. 4;
Meurling, 1962). Sometimes isolated axons penetrate the boundary membrane and take an irregular course between the intermedia cells. More
often, and particularly in the neural processes, groups of neurosecretory