8. UROPHYSIS AND CAUDAL NEUROSECRETORY SYSTEM
407
projecting ventromedially to a capillary plexus at the base of the spinal
cord. The diverse patterns even within the narrow confines of this general
picture (Fig. 6) have been delineated by Fridberg (1962b, 1963). In
some species additional axonlike processes extend to the ventrolateral
region of the meninx. The cells are much more extensively distributed in
elasmobranchs than in teleosts.
B. Evidence for Caudal Neurosecretory Neurons in
Other Vertebrate Groups
The fundamental point to be remembered regarding the existence of
a caudal neurosecretory system in fishes other than elasmobranchs and
teleosts is the total absence of a urophysis from all nonteleostean fishes.
However, in view of the existence of a well-developed, albeit diffuse,
neurohemal area in elasmobranchs, and occasional diffuse neurohemal
areas even when a formed urophysis exists in some isospondylous teleosts, the lack of a urophysis does not mean absence of the caudal
neurosecretory system.
1. CYCLOSTOMES
No definite neurosecretory cells have been identified in the living
agnathans (Peyrot, 1964), although some confusion has arisen from the
misinterpretation of other large neurons as neurosecretory.
2. HOLOCEPHALANS
Cursory examination of a Pacific coast ratfish species, Hydrolugus
colliei (Bern and Takasugi, 1962), has not revealed any situation comparable to the caudal apparatus so prominent in the major chondrichthyean subclass, the elasmobranchs. In view of the presence of such cells in
elasmobranchs and teleosts, it is surprising that the ratfishes, which arose
from the same primary stock leading to ancestral chondrichthyeans and
osteichthyeans, should be devoid of these elements. A more detailed
analysis of the caudal spinal cord with the use of electron microscopy
seems indicated.
3. LOWER ACTINOPTERYGIANS
In his pioneer studies, Speidel (1922) indicated the presence of caudal
neurosecretory cells in the garpike, Lepisosteus osteus, and Fridberg
(1962a) has found these cells in Polypterus.
407
projecting ventromedially to a capillary plexus at the base of the spinal
cord. The diverse patterns even within the narrow confines of this general
picture (Fig. 6) have been delineated by Fridberg (1962b, 1963). In
some species additional axonlike processes extend to the ventrolateral
region of the meninx. The cells are much more extensively distributed in
elasmobranchs than in teleosts.
B. Evidence for Caudal Neurosecretory Neurons in
Other Vertebrate Groups
The fundamental point to be remembered regarding the existence of
a caudal neurosecretory system in fishes other than elasmobranchs and
teleosts is the total absence of a urophysis from all nonteleostean fishes.
However, in view of the existence of a well-developed, albeit diffuse,
neurohemal area in elasmobranchs, and occasional diffuse neurohemal
areas even when a formed urophysis exists in some isospondylous teleosts, the lack of a urophysis does not mean absence of the caudal
neurosecretory system.
1. CYCLOSTOMES
No definite neurosecretory cells have been identified in the living
agnathans (Peyrot, 1964), although some confusion has arisen from the
misinterpretation of other large neurons as neurosecretory.
2. HOLOCEPHALANS
Cursory examination of a Pacific coast ratfish species, Hydrolugus
colliei (Bern and Takasugi, 1962), has not revealed any situation comparable to the caudal apparatus so prominent in the major chondrichthyean subclass, the elasmobranchs. In view of the presence of such cells in
elasmobranchs and teleosts, it is surprising that the ratfishes, which arose
from the same primary stock leading to ancestral chondrichthyeans and
osteichthyeans, should be devoid of these elements. A more detailed
analysis of the caudal spinal cord with the use of electron microscopy
seems indicated.
3. LOWER ACTINOPTERYGIANS
In his pioneer studies, Speidel (1922) indicated the presence of caudal
neurosecretory cells in the garpike, Lepisosteus osteus, and Fridberg
(1962a) has found these cells in Polypterus.