132
A. M. PERKS
axons pass through gaps in the reticular membrane and run between the
intermedia cells or within the connective tissue which divides the cells
into cords. Other species show a greater penetration by the pars nervosa
(Fig, 4). Etmopterus spinax is essentially similar to Squalus acanthias,
except that the pars nervosa is covered by a connective tissue membrane
which contains extensive fenestrations in the posterior region of the gland
( Meurling, 1963). Here, nerve axons penetrate the pars intermedia; some
are neurosecretory and form occasional terminal swellings against the
cells, but others carry no neurosecretion and terminate in a nerve plexus
around the blood sinusoids of the pars intermedia. In Scyliorhinus caniculus, S . stellaris, and Mustelus mustelus, the neural lobe is still recognizable, but there is no membrane and no typical, continuous neural lobe
blood plexus to limit its boundary ( Meurling, 1962, 1967a). In Scyliorhinus caniculus nerve fibers penetrate between the intermedia cells, and
neurosecretion of high sulfur content is found packing closely around
these cells (Perks, 1959). Electron microscopy has shown that the
neurosecretory nerves make close contacts, suggestive of synapses, with
the intermedia cells (Mellinger, 1963b). In Scyliorhinus stellaris (syn.
Scyllium stellure) a high proportion of the neurosecretory axons enter the
pars intermedia and secretion is stored everywhere between the cells
( Scharrer, 1952). Electron microscopy has shown that the neurosecretory
fibers can be divided into two types. The first, Type A, contains large
electron dense vesicles and makes contact with the “synthetic” region of
the intermedia cells. The second, Type B, contains small, irregular neurosecretory elements and terminates on the “storage and release” region of
the cells. This suggests the possibility of a separate control of synthesis
and release from the intermedia cells (Knowles, 1963, 1965). In Pristiurus
melanostomas there is an intimate fusion of neural and intermediate
components, and in many places a neural lobe is no longer distinguishable; no neural lobe plexus has been described (Meurling, 1967a). In the
skates and rays (Raia batis, R. oxyrhynchus, R. fullonica, R. radiata, R.
clavata, Dasyatis sp., and Torpedo sp.) there is complete fusion of neural
and intermediate elements. The only region which could possibly be
termed a neural lobe is the small anterior area of the infundibular floor,
where the preoptico-hypophysial tract enters the neurointennediate lobe
( Bargmann, 1955; Meurling, 1962, 1967a). There is no neural lobe plexus,
but blood vessels are found close to the ventricular surface, and the entire neurointermediate lobe contains a rich plexus of sinuses (Meurling,
1967a). A combination of light and electron microscopy has suggested
that the bundles of nerve fibers which penetrate between the strands of
intermedia cells in Raia clavata and Trygon pastinaca are made up of
both neurosecretory axons and nerve fibers which do not carry such
A. M. PERKS
axons pass through gaps in the reticular membrane and run between the
intermedia cells or within the connective tissue which divides the cells
into cords. Other species show a greater penetration by the pars nervosa
(Fig, 4). Etmopterus spinax is essentially similar to Squalus acanthias,
except that the pars nervosa is covered by a connective tissue membrane
which contains extensive fenestrations in the posterior region of the gland
( Meurling, 1963). Here, nerve axons penetrate the pars intermedia; some
are neurosecretory and form occasional terminal swellings against the
cells, but others carry no neurosecretion and terminate in a nerve plexus
around the blood sinusoids of the pars intermedia. In Scyliorhinus caniculus, S . stellaris, and Mustelus mustelus, the neural lobe is still recognizable, but there is no membrane and no typical, continuous neural lobe
blood plexus to limit its boundary ( Meurling, 1962, 1967a). In Scyliorhinus caniculus nerve fibers penetrate between the intermedia cells, and
neurosecretion of high sulfur content is found packing closely around
these cells (Perks, 1959). Electron microscopy has shown that the
neurosecretory nerves make close contacts, suggestive of synapses, with
the intermedia cells (Mellinger, 1963b). In Scyliorhinus stellaris (syn.
Scyllium stellure) a high proportion of the neurosecretory axons enter the
pars intermedia and secretion is stored everywhere between the cells
( Scharrer, 1952). Electron microscopy has shown that the neurosecretory
fibers can be divided into two types. The first, Type A, contains large
electron dense vesicles and makes contact with the “synthetic” region of
the intermedia cells. The second, Type B, contains small, irregular neurosecretory elements and terminates on the “storage and release” region of
the cells. This suggests the possibility of a separate control of synthesis
and release from the intermedia cells (Knowles, 1963, 1965). In Pristiurus
melanostomas there is an intimate fusion of neural and intermediate
components, and in many places a neural lobe is no longer distinguishable; no neural lobe plexus has been described (Meurling, 1967a). In the
skates and rays (Raia batis, R. oxyrhynchus, R. fullonica, R. radiata, R.
clavata, Dasyatis sp., and Torpedo sp.) there is complete fusion of neural
and intermediate elements. The only region which could possibly be
termed a neural lobe is the small anterior area of the infundibular floor,
where the preoptico-hypophysial tract enters the neurointennediate lobe
( Bargmann, 1955; Meurling, 1962, 1967a). There is no neural lobe plexus,
but blood vessels are found close to the ventricular surface, and the entire neurointermediate lobe contains a rich plexus of sinuses (Meurling,
1967a). A combination of light and electron microscopy has suggested
that the bundles of nerve fibers which penetrate between the strands of
intermedia cells in Raia clavata and Trygon pastinaca are made up of
both neurosecretory axons and nerve fibers which do not carry such