REGULATION'OF FORM AND STRUCTURES 103
is usually formed from cartilage, but it m'ayhave an entirely
—
mémbranous origin.
,
_
»
.
'
-
-,
The formation of bone in the membrane above the head
has come as a direct response to the pressure of the surrounding medium.” Originally, as explainedinf a previous
chapter, the bone was probably laid dOWn in the headand
_
body simply.às an exci‘etiOn, and its arrangement later into
denite forms and positions has ”come just from the outer
pressure. , Then it came
as a protection to the brain.
Another factor of importance in determining .the shape
of the head andits structure is the inherited specic balance.
The normal tenderïcy is
the head to grow forward
horizontally from the trunk, but very frequently it is bent
dOWHwards on the vertebral axis, which means a restriction,
of the space for the organs and structüres under the head,
and thus a special rearrangement of the latter. This
depression of the skull ismostly seen in the advanced (")r
specialised groups of Teleosts, which have
Open
‘
communication of the air—bladder to the gullet and in which
the head is massive in_comparison with-fçhe'caudal region
,
to “…the balance being
'
'
concentration of the substance
,;PefhäpS the most.— important factor indeteriirng the
shape of the head, and even of the body 1nmany cases,rhas
been the development of the Various
with the mouth. From the greatvarietyof these, structures
we can see that this has been a very difcult matter for the
-shes.
There are three different kinds 'of môuths and
each naturally has led to differences in the skull above and
,
the arches behind. The relations of the various parts of
'thisf'œmplicated region are shown in the accompanyig
diagram. of the
in the adult Herring“ (Fig.’25).
In some _‘fways; this isthe -most.advancedztype of month, but
_it lerid_s itself , _ also
the 'siinpler, though
not nece3sarilyÿmoreprimitive types.:
'
:
—
—
_.
?
: 7
_
_
It
be seen that
or?
loWer
—
_] aw is cOnnected tothe foot of the
(thus to the
is usually formed from cartilage, but it m'ayhave an entirely
—
mémbranous origin.
,
_
»
.
'
-
-,
The formation of bone in the membrane above the head
has come as a direct response to the pressure of the surrounding medium.” Originally, as explainedinf a previous
chapter, the bone was probably laid dOWn in the headand
_
body simply.às an exci‘etiOn, and its arrangement later into
denite forms and positions has ”come just from the outer
pressure. , Then it came
as a protection to the brain.
Another factor of importance in determining .the shape
of the head andits structure is the inherited specic balance.
The normal tenderïcy is
the head to grow forward
horizontally from the trunk, but very frequently it is bent
dOWHwards on the vertebral axis, which means a restriction,
of the space for the organs and structüres under the head,
and thus a special rearrangement of the latter. This
depression of the skull ismostly seen in the advanced (")r
specialised groups of Teleosts, which have
Open
‘
communication of the air—bladder to the gullet and in which
the head is massive in_comparison with-fçhe'caudal region
,
to “…the balance being
'
'
concentration of the substance
,;PefhäpS the most.— important factor indeteriirng the
shape of the head, and even of the body 1nmany cases,rhas
been the development of the Various
with the mouth. From the greatvarietyof these, structures
we can see that this has been a very difcult matter for the
-shes.
There are three different kinds 'of môuths and
each naturally has led to differences in the skull above and
,
the arches behind. The relations of the various parts of
'thisf'œmplicated region are shown in the accompanyig
diagram. of the
in the adult Herring“ (Fig.’25).
In some _‘fways; this isthe -most.advancedztype of month, but
_it lerid_s itself , _ also
the 'siinpler, though
not nece3sarilyÿmoreprimitive types.:
'
:
—
—
_.
?
: 7
_
_
It
be seen that
or?
loWer
—
_] aw is cOnnected tothe foot of the
(thus to the