116
A. M. PERKS
thickness, and this might be regarded as the pars nervosa. Here, there is
a relatively dense accumulation of neurosecretion. The neurosecretory
droplets are closely associated with a rich vascular bed which penetrates
into the dorsal surface of the neural lobe. This plexus drains into the general circulation, and it could serve as a route for the delivery of neurohypophysial peptides to the tissues of the body.
There is no good evidence for the passage of neurosecretory material
through the ventral wall of the neural lobe to the poorly differentiated
adenohypophysis. Herlant ( 1954) has suggested that the poor differentiation of the adenohypophysis may result from this lack of neural contact.
The ventral wall of the neural lobe, which faces the islets of adenohypophysial tissue, is poorly supplied with either nerve terminations or
neurosecretory granules (Adam, 1963a). In many specimens of Myxine
and Polistotrema the two regions of the pituitary are separated by a thick
layer of connective tissue (Retzius, 1895; Olsson, 1959). This is penetrated only by a few, small blood vessels (Gorbman et al., 1963). However, Matty (1960a,b) has described specimens of Myxine in which there
is intimate contact between the caudal region of the neural lobe and the
adenohypophysis. This apparent contradiction of a number of other workers could result from individual variation, but it could also be explained
by Adam’s observation (1963a) that the connective tissue septum becomes thinned in specimens over 30 cm in length. If this degree of contact increased with age and length, it is possible that the neurohypophysis
could come to influence the adenohypophysis in later life. However,
Matty could find no evidence for the passage of neurosecretory droplets
into the adenohypophysial tissues even though contact existed. Clearly,
it is not possible to discount interactions between the two regions of the
pituitary, but the present consensus is against their existence. It is worth
noting that Gorbman et al. (1963) have pointed out that the neurohypophysial circulation of the hagfish is well adapted to the delivery of
hormones to the general circulation.
2. PETROMYZONIFORMES
The neurohypophysial system of the lamprey is markedly different
from that of the hagfish. Unlike the hagfish, the lampreys do not develop
a clear infundibular process (Fig. 2). Nevertheless, studies of Petromyzon
marinus, Lampetra lumottei, and L. planeri have shown that a complete
neurohypophysial system is present in the slightly thickened floor of the
hypothalamus ( de Beer, 1926; Tilney, 1937; Bargmann, 1953; Scharrer,
1953-1954; Sterba, 1961; W. H. Sawyer, 1961; Oztan and Gorbman,
1960a,b). The preoptic nucleus is well marked, although less prominent
A. M. PERKS
thickness, and this might be regarded as the pars nervosa. Here, there is
a relatively dense accumulation of neurosecretion. The neurosecretory
droplets are closely associated with a rich vascular bed which penetrates
into the dorsal surface of the neural lobe. This plexus drains into the general circulation, and it could serve as a route for the delivery of neurohypophysial peptides to the tissues of the body.
There is no good evidence for the passage of neurosecretory material
through the ventral wall of the neural lobe to the poorly differentiated
adenohypophysis. Herlant ( 1954) has suggested that the poor differentiation of the adenohypophysis may result from this lack of neural contact.
The ventral wall of the neural lobe, which faces the islets of adenohypophysial tissue, is poorly supplied with either nerve terminations or
neurosecretory granules (Adam, 1963a). In many specimens of Myxine
and Polistotrema the two regions of the pituitary are separated by a thick
layer of connective tissue (Retzius, 1895; Olsson, 1959). This is penetrated only by a few, small blood vessels (Gorbman et al., 1963). However, Matty (1960a,b) has described specimens of Myxine in which there
is intimate contact between the caudal region of the neural lobe and the
adenohypophysis. This apparent contradiction of a number of other workers could result from individual variation, but it could also be explained
by Adam’s observation (1963a) that the connective tissue septum becomes thinned in specimens over 30 cm in length. If this degree of contact increased with age and length, it is possible that the neurohypophysis
could come to influence the adenohypophysis in later life. However,
Matty could find no evidence for the passage of neurosecretory droplets
into the adenohypophysial tissues even though contact existed. Clearly,
it is not possible to discount interactions between the two regions of the
pituitary, but the present consensus is against their existence. It is worth
noting that Gorbman et al. (1963) have pointed out that the neurohypophysial circulation of the hagfish is well adapted to the delivery of
hormones to the general circulation.
2. PETROMYZONIFORMES
The neurohypophysial system of the lamprey is markedly different
from that of the hagfish. Unlike the hagfish, the lampreys do not develop
a clear infundibular process (Fig. 2). Nevertheless, studies of Petromyzon
marinus, Lampetra lumottei, and L. planeri have shown that a complete
neurohypophysial system is present in the slightly thickened floor of the
hypothalamus ( de Beer, 1926; Tilney, 1937; Bargmann, 1953; Scharrer,
1953-1954; Sterba, 1961; W. H. Sawyer, 1961; Oztan and Gorbman,
1960a,b). The preoptic nucleus is well marked, although less prominent