2. THE NEUROHYPOPHYSIS
113
vasopressin, in the pig, opened up the possibility of species variability in
neurohypophysial peptides (Popenoe et al., 1952). It gave new significance to Heller’s early demonstration ( 1941b ) that lower vertebrate pituitaries, including those of teleost fish, contained a neurohypophysial principle which was particularly potent in promoting water reabsorption in
the frog. Soon, this “water balance factor” was shown to be arginine
vasotocin, a molecule containing moieties from both oxytocin and arginine
vasopressin ( W. H. Sawyer et al., 1959), and it became clear that the
lower vertebrates, including the fish, contained a fascinating family tree
of interrelated principles (Perks et al., 1960; Acher et al., 1962). Perhaps
the least satisfactory phase of neurohypophysial studies has been in the
elucidation of the function of neurohypophysial principles in the metabolism of fish. Although concepts derived from mammalian studies have
been useful, they may have clouded our view to some extent, since it is
probable that the peptides have new and, as yet, undiscovered functions
in aquatic vertebrates. Perhaps further work will allow the fish to introduce yet new ideas into mammalian physiology, this time new ideas concerning function. In this vein, it is particularly interesting that recent
work has suggested that arginine vasotocin may be present in the mammal during its term of “aquatic” existence, i.e., during fetal development
in the uterus ( Vizsolyi and Perks, 1968).
11. THE CYCLOSTOMES
A. The Structure of the Neurohypophysis of the Cyclostomes
The hagfish ( Myxiniformes) and the lampreys (Petromyzoniformes)
hold a place of special importance in comparative studies, since they are
the only living representatives of the first vertebrate class to be found
in the fossil record. Already, they possess a relatively well developed
neurohypophysial system ( Gorbman and Bern, 1964), with only general
tendencies which might appear to be primitive. This implies that comparative studies can examine only the final molding of the system, and its
origins will remain obscure. However, the fact that it exists in both the
two living groups of cyclostomes, which may have originated separately
from ancestral Ostracoderms (see Heintz, 1963), suggests that it was
present also in these common ancestors. If this diphyletic origin is correct,
the hagfish and the lampreys may not be as closely related as is often
assumed. This consideration, together with their long history of separate
development, their clear specializations, and their degenerate features,
113
vasopressin, in the pig, opened up the possibility of species variability in
neurohypophysial peptides (Popenoe et al., 1952). It gave new significance to Heller’s early demonstration ( 1941b ) that lower vertebrate pituitaries, including those of teleost fish, contained a neurohypophysial principle which was particularly potent in promoting water reabsorption in
the frog. Soon, this “water balance factor” was shown to be arginine
vasotocin, a molecule containing moieties from both oxytocin and arginine
vasopressin ( W. H. Sawyer et al., 1959), and it became clear that the
lower vertebrates, including the fish, contained a fascinating family tree
of interrelated principles (Perks et al., 1960; Acher et al., 1962). Perhaps
the least satisfactory phase of neurohypophysial studies has been in the
elucidation of the function of neurohypophysial principles in the metabolism of fish. Although concepts derived from mammalian studies have
been useful, they may have clouded our view to some extent, since it is
probable that the peptides have new and, as yet, undiscovered functions
in aquatic vertebrates. Perhaps further work will allow the fish to introduce yet new ideas into mammalian physiology, this time new ideas concerning function. In this vein, it is particularly interesting that recent
work has suggested that arginine vasotocin may be present in the mammal during its term of “aquatic” existence, i.e., during fetal development
in the uterus ( Vizsolyi and Perks, 1968).
11. THE CYCLOSTOMES
A. The Structure of the Neurohypophysis of the Cyclostomes
The hagfish ( Myxiniformes) and the lampreys (Petromyzoniformes)
hold a place of special importance in comparative studies, since they are
the only living representatives of the first vertebrate class to be found
in the fossil record. Already, they possess a relatively well developed
neurohypophysial system ( Gorbman and Bern, 1964), with only general
tendencies which might appear to be primitive. This implies that comparative studies can examine only the final molding of the system, and its
origins will remain obscure. However, the fact that it exists in both the
two living groups of cyclostomes, which may have originated separately
from ancestral Ostracoderms (see Heintz, 1963), suggests that it was
present also in these common ancestors. If this diphyletic origin is correct,
the hagfish and the lampreys may not be as closely related as is often
assumed. This consideration, together with their long history of separate
development, their clear specializations, and their degenerate features,