THE DEVELOPMENT OF FISHES
69Ç
absorption of the yolk—food, and the formati0n of the muscles
and internal organs.
'
.
-
…
’
Specialisation or organisation;ha$ thus begun, audit
cannot be Without some pregnant meaning that the rst
part or organ to be formed isthe central nervous system.
In some mysterious way a furrow appears alog-thet0piof
the blastodisc, and the sides sWell up and appr0acheach
Other until they meet oVer the furrow, then fusing tOform a
canal Or it may be at rst a solid axis.
anterior end
,
several swelligs soon appear,denoting the vesicles of the“
brain.
-
—
_
‘The stage now reached may be said to be theposseSSion
only of the shes and higher animals (Craniata), yet signs
; of 'it- are not wanting amongst still lowerforms (Cepha;
IOGhordata and Urochordata). - And going back still further,
the tWo-layered stage or gastrula is common to-many Inverte— '
bràtés.
In this way, therefore, the past hiStory is being
_
but the , gap {is very great,and some’zoologists
believe‘ that'here at any rate nature has broken With tradition
_
and completely reversed’ the order of. earlier proceedings.
The larva of the worm, it is supposéd, has tured.ùpside
_
forwardinstead of bâchWard, the nerve
_
cord becomes dorsal inStead of ventral, Whilst the heart
the nerve cord «and—-alimætärÿïçànal
'
instead of above. “Further, an entirely-new mouthis formed;
'
*
As We shall See, the forrhati0n of a mouth has been
“difcult problem f0rthgä=shes; —
..
®
'
=:
…
Â
'
—
This revolution the0ry-cann0t, of Course, be proved,but
it— provides us With the
of theimmense
:
change in form and structure We nd among the_shes and._ _
Vertebraté‘ë. 7=The
has come,asW€re,lnt0 a
new World, found it
hasthen expanded In
”many “different
: :
.:'î ï* =;
'
_—î f _ …
’
;
>Whilst_the dorSal nerve‘4çordf is'çformÿing, ÏthegglgwàärÏflërj— ,
=
.
“of cells—, îs €hgaged" in
amassofmusdeœlls
i -
—,
alongyeach (side. andthesesooncometogew1ngroupS ; :
probably - alreadyî ùder;_ÿ,.theîÿdiiéfctins:Inuence0fthe
t'
‘1
.
7