ioo
‘
"THE 'BIOLOGY OF, FISHES
'
‘
'
';_
… _,
-
3. FORMATION OF THE HEAD
_
'
In Houssay’s experiments the head of a yieldingbody was
:
attened above and compressed from side to side on being
drawn through the water.
'
This We can readily understand,
'
and it indicates ‘one of themain factors Ï_inthe formation of
,
the head, namely, the pressure of the
,
If we compare the Herring or
f and 2) With a:
.,
form like the Lumpsûcker, Cyclopt€rus (Pl. XIV), xvhiCh
frOm—the beginning moves but little through the
;
,,rhas—_a
head,we see at
a
*
,—
dierencem0vement makes; :
_
—
'
»
,
,
a Clupeidlarvajust afterithas hatched
,Ï
very Closely to the
À?
_,
;: plast1cbodvofHeasæys experiments_e,,'i It, -is: founded» , ,,
'alm@stSiâhencalWlththebramcunedmund inêfr0nfand —
.
nomeuthbdowIntemallyhasplacucHynostrucure ;,
? *
th1ddblafg……y
ndbkwdtwdthbdybutthfæb
.
"
,
f
may fÿeë_rtainly be
as
printivë (Wells)
M
Th‘Ë'
€} hearing ’,?îorgan
statocyst is representedonly by'a
_
a
cartilagi'nous nucleus, so that even the cranium proper, the
part—behind the eyes, has still nothing solid about it. ‘
—
,
During the larval and postlafval periode this head
,,
-
…-graduay becomes nat—rw, andpomtedThebramm
:
:!
forced back,» between andbeh1ndtheeyesmto1ecramum !?
Asarulethesephen0memareSlmpîyreferredto as growth
}
thatthedel‘teshapeatta1ned1s due to the pressure of the
.
Î<
-
GroWth if unhindered
dimensions, as in the case of
:
*; henœ: the actual form and therewith
…
underlying—strucwre :aretdetermined by
.ff
surrounding pressure.
,
_
,
,
f
,,
,
—.
.
—; In
We have the Elasmob'ranch enibrÿo
,
_î which develOps under sheltered conditions
.