102
THE, BIOLOGY, OF FISHES
it» ispressed against the trunk. But the most interesting
—
thing is the position and nature of the bones. The upper
…
part or roof of the'skull is formed by the long membranous
frontals (f) with very small parietals @) and supraoccipital
'
posteriorly (ss). Only the last is a cartilage bone.
*
_
-
The distinction between these two different ways of
forming osseous tissue is a very important-one.
It has been
assumed by many that cartilage preceded bone, and conse—5
quently, if bone forms in membrane without any appearance
of cartilage, the latter must somehow have disappeared. But
-
as a rule the possibility of bone forming directly in membrane
without the f aid of cartilage is now generally recognised,
and we see here in the Clupeids that the roof of the skull
is formed entirely of thin membrane boues”. Theblder idea
was based to a large extent on the,belief that
shes, Elasmobranchs, precededthe bony shes,
the
term “ Teleosts.’_’ But as a matter of fact—, the earlieSt 'shes
had
plates ’on the head of a membranous of derinal
nature just like the modern Teleosts, and What have been
taken to be the earliest ElasmObraçhs were also-provided
'
similar ossications.
It is unnecessary to believe,»
'
therefore, that Car—tilage preceded bone. It seems, indeed,
,
,
that cartilage“ develops‘ mainly if not entirely in places not
exposed to great pressures, and where muscular tissue
-
(mesoderm) is present. This cartilage
oi“, _may, not
"
become o’ssied by the in-wanderingof—-Cèlls‘.from_itheÿgsur—_
or by the deposition ofîÏ_calcium Êsà‘lts
"
the sh or
'
î
,
_
‘r‘emarkable distribu'
ÎIËhüs;thé neural and hæmal spines are
Ïmo'stly-Âformed fr'-Om bone
-.
laid down in the membrane
Ï
the radialia andthe ribs are rst '
'"
of all formed of cartilage, derived probably from the
»;
'
cells.— Similarlÿ",
region the parasphenoid” (pf),
E
1
which formsthe strong supporting. axis of the baSe of the
;
asku1l», is really a membrane bone…though‘it is formed under—j
'
,
neath and closeto the primordial cartilage-. The palatinè '
,
"
THE, BIOLOGY, OF FISHES
it» ispressed against the trunk. But the most interesting
—
thing is the position and nature of the bones. The upper
…
part or roof of the'skull is formed by the long membranous
frontals (f) with very small parietals @) and supraoccipital
'
posteriorly (ss). Only the last is a cartilage bone.
*
_
-
The distinction between these two different ways of
forming osseous tissue is a very important-one.
It has been
assumed by many that cartilage preceded bone, and conse—5
quently, if bone forms in membrane without any appearance
of cartilage, the latter must somehow have disappeared. But
-
as a rule the possibility of bone forming directly in membrane
without the f aid of cartilage is now generally recognised,
and we see here in the Clupeids that the roof of the skull
is formed entirely of thin membrane boues”. Theblder idea
was based to a large extent on the,belief that
shes, Elasmobranchs, precededthe bony shes,
the
term “ Teleosts.’_’ But as a matter of fact—, the earlieSt 'shes
had
plates ’on the head of a membranous of derinal
nature just like the modern Teleosts, and What have been
taken to be the earliest ElasmObraçhs were also-provided
'
similar ossications.
It is unnecessary to believe,»
'
therefore, that Car—tilage preceded bone. It seems, indeed,
,
,
that cartilage“ develops‘ mainly if not entirely in places not
exposed to great pressures, and where muscular tissue
-
(mesoderm) is present. This cartilage
oi“, _may, not
"
become o’ssied by the in-wanderingof—-Cèlls‘.from_itheÿgsur—_
or by the deposition ofîÏ_calcium Êsà‘lts
"
the sh or
'
î
,
_
‘r‘emarkable distribu'
ÎIËhüs;thé neural and hæmal spines are
Ïmo'stly-Âformed fr'-Om bone
-.
laid down in the membrane
Ï
the radialia andthe ribs are rst '
'"
of all formed of cartilage, derived probably from the
»;
'
cells.— Similarlÿ",
region the parasphenoid” (pf),
E
1
which formsthe strong supporting. axis of the baSe of the
;
asku1l», is really a membrane bone…though‘it is formed under—j
'
,
neath and closeto the primordial cartilage-. The palatinè '
,
"