190
A. M. PERKS
typical of the tetrapods. The median eminence and pituitary portal system
is a common feature of the neurohypophysial system from the level of the
primitive hagfish up through the vertebrate tree. In its early stages of
development it appears to possess connections to the neurointemediate
lobe (Myxiniformes, some Elasmobranchii). We are faced less with the
problem of how and why it developed in the vertebrates, and more with
the question of why it is inconspicuous in the lampreys and absent in the
teleosts. The lampreys do not have a marked portal system, but the
evidence presented here favors the presence of a small neural and neurovascular connection between the preoptic nucleus and the adenohypophysis; its effects may well be aided by the large sinuses which lie
between the pars nervosa and the rest of the pituitary. The teleosts have
developed direct neural contacts with the adenohypophysis, and it is
possible that the portal system became obsolete and was lost. However,
the importance of both neural and vascular connections between the
neurohypophysis and the adenohypophysis suggests that the neurohypophysis may well mediate a control over the pars intermedia and over
other adenohypophysial regions as well.
Arginine vasotocin has been demonstrated in most fish which have
been examined, and it is strongly suspected in most others. Although
4 Gln, 8 Leu
(oxytocin and/or mesotocin
4 Gln, 8 Ile
4 Ser, 8 Ile
(isotocin)
4 Gln. 8 Leu
4 Ser, 8 Gln
(glumitocin)
Actinopterygians
Lungfish
4 Gln, 8 Arg
Osteichthyes
Agnatha
Fig. 12. The distribution of neutral neurohypophysial principles throughout different classes of fish. The cyclostomes are exceptional in possessing only the basic
principle, arginine vasotocin (supplied by the kindness of Dr. W. H. Sawyer).
A. M. PERKS
typical of the tetrapods. The median eminence and pituitary portal system
is a common feature of the neurohypophysial system from the level of the
primitive hagfish up through the vertebrate tree. In its early stages of
development it appears to possess connections to the neurointemediate
lobe (Myxiniformes, some Elasmobranchii). We are faced less with the
problem of how and why it developed in the vertebrates, and more with
the question of why it is inconspicuous in the lampreys and absent in the
teleosts. The lampreys do not have a marked portal system, but the
evidence presented here favors the presence of a small neural and neurovascular connection between the preoptic nucleus and the adenohypophysis; its effects may well be aided by the large sinuses which lie
between the pars nervosa and the rest of the pituitary. The teleosts have
developed direct neural contacts with the adenohypophysis, and it is
possible that the portal system became obsolete and was lost. However,
the importance of both neural and vascular connections between the
neurohypophysis and the adenohypophysis suggests that the neurohypophysis may well mediate a control over the pars intermedia and over
other adenohypophysial regions as well.
Arginine vasotocin has been demonstrated in most fish which have
been examined, and it is strongly suspected in most others. Although
4 Gln, 8 Leu
(oxytocin and/or mesotocin
4 Gln, 8 Ile
4 Ser, 8 Ile
(isotocin)
4 Gln. 8 Leu
4 Ser, 8 Gln
(glumitocin)
Actinopterygians
Lungfish
4 Gln, 8 Arg
Osteichthyes
Agnatha
Fig. 12. The distribution of neutral neurohypophysial principles throughout different classes of fish. The cyclostomes are exceptional in possessing only the basic
principle, arginine vasotocin (supplied by the kindness of Dr. W. H. Sawyer).