408
HOWARD A. BERN
4. DIPNOANS
On the other hand, representatives of the sarcopterygian group of
Osteichthyes do not appear to possess Dahlgren cells, although some
larger neurons in the caudal spinal cord have been seen (Bern and
Takasugi, 1962). As with the ratfishes, detailed reexamination is called
for.
5. TETRAPODS
There is no good evidence for the occurrence of caudal neurosecretory
cells in tetrapod spinal cords. Speidel, in 1922, found no indication of
such cells in tailed amphibians (see also Bogoraze, 1968), and the lumbar
motor neuron analogy suggested by Sano (1954) is highly doubtful
( Ghosh et al., 1962).
C. Evolution of the Caudal Neurosecretory System
The evolution of the caudal neurosecretory system can be visualized
as beginning with the primitive elasmobranch condition with diffusely
distributed neurosecretory neurons giving rise to a diffuse neurohemal
zone. There is no neurosecretory tract as such, inasmuch as the short
processes of the Dahlgren cells generally project ventrally to attain immediate contact with the capillary bed. This anatomic picture (Fig. 6)
can be considered as a genuinely primitive condition; in most teleosts the
lengthening of the axons and the localization of the capillary bed lead to
an organized, often lobate, urophysis. However, more primitive teleosts
(clupeiform), as well as some more advanced forms (syngnathiform,
anguilliform ) , often have a diffuse urophysial zone, barely detectable
externally. It is also possible, of course, that the more &se condition is
a secondary evolutionary development.
The primary determinant of the external configuration of the neurohemal area would seem to be the vertebral column. In order for a lobate
urophysis to exist, there must be space to accommodate it. Isospondylous
vertebral columns, with a gradually tapering spinal cord housed in vertebral elements of gradually decreasing size, are to be distinguished from
the more advanced type of caudal skeleton which includes a terminal
vertebral centrum modified into a urostyle. In the urostylar centrum is a
cavity-the urophysial fossa-which will accommodate a lobate urophysis.
HOWARD A. BERN
4. DIPNOANS
On the other hand, representatives of the sarcopterygian group of
Osteichthyes do not appear to possess Dahlgren cells, although some
larger neurons in the caudal spinal cord have been seen (Bern and
Takasugi, 1962). As with the ratfishes, detailed reexamination is called
for.
5. TETRAPODS
There is no good evidence for the occurrence of caudal neurosecretory
cells in tetrapod spinal cords. Speidel, in 1922, found no indication of
such cells in tailed amphibians (see also Bogoraze, 1968), and the lumbar
motor neuron analogy suggested by Sano (1954) is highly doubtful
( Ghosh et al., 1962).
C. Evolution of the Caudal Neurosecretory System
The evolution of the caudal neurosecretory system can be visualized
as beginning with the primitive elasmobranch condition with diffusely
distributed neurosecretory neurons giving rise to a diffuse neurohemal
zone. There is no neurosecretory tract as such, inasmuch as the short
processes of the Dahlgren cells generally project ventrally to attain immediate contact with the capillary bed. This anatomic picture (Fig. 6)
can be considered as a genuinely primitive condition; in most teleosts the
lengthening of the axons and the localization of the capillary bed lead to
an organized, often lobate, urophysis. However, more primitive teleosts
(clupeiform), as well as some more advanced forms (syngnathiform,
anguilliform ) , often have a diffuse urophysial zone, barely detectable
externally. It is also possible, of course, that the more &se condition is
a secondary evolutionary development.
The primary determinant of the external configuration of the neurohemal area would seem to be the vertebral column. In order for a lobate
urophysis to exist, there must be space to accommodate it. Isospondylous
vertebral columns, with a gradually tapering spinal cord housed in vertebral elements of gradually decreasing size, are to be distinguished from
the more advanced type of caudal skeleton which includes a terminal
vertebral centrum modified into a urostyle. In the urostylar centrum is a
cavity-the urophysial fossa-which will accommodate a lobate urophysis.