84
THE BIOLOGY or FISHE8
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necessarily the same as {hat in which it was born, All
shes pass through several different environments in the
early stages and the resultant structure is a summation of
'
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responsive co-ordinations. The most interesting period,
and the one from which we may hope to obtain the best
information regarding the past history, is therefore the early
‘
stage when the structures aréÏf-ornng.
,
Î
It has been Shown in the preceding "chapter, that the
;v€—rtebral
arises under the Ï combined inuence of
:
the" movements andthe outer conditions. But the nature
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of the movements depends naturallyon‘gth6 balance or poise
‘; "?
of the sh. If the sh wishes to raiseÏits head, the muscles
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along the back are called into play}. And if the sh is too
heàVy at the headfend,
be a pull on the tail end
=
occurs in many sheSand
leads to
of aÎ-ïcharacteristic structurein shes, .
the;tail. How the latter is
ways under-the
inuence of - the movements, will beunderstoodg.W-ith the '
:
aidof'the accompanying gures.
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Taking one
—we nd :;that. the
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is
and slightly ossiéd) _7
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2) Then the. muscle along the top seems to
break away from
connected with the
raising of the head—and croSses the notochord somedistance
’
Hereîh
cells proceed to form
a number of cartilages, and with these
LÏ“then bends
tip of the ossied‘notochord (urostyle)
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’int©: Whät‘ is called, the. hetèrocerçall PbsiÏiôn-* Under the
.,
”strain “at,
Late
broken
Side”,
‘ufoneurals.
the
at thei—tipÏbe—i—ng constricted
'
is
or“: atfo‘phies, and theÿüp- of, the notochordalso
.
collapses to -form a. uroneural or two uroneurals. ; The
number“of cartiläges below (hypu—r—als) may—.vary
tWo
-j'
to. eight, as‘ in the gure,_and in. the‘nal stages Several splint
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:
b0nes (epurals) develop above
the upper rays of
;
*-I_t*is__ in“ this way that-_ the. apparently sym-.
‘
;?
tail is formed.
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