”70
THE BIOLOGY OF FISHES—
_:
plainly seen in the young embryo (Fig. 13, 5). The embryo
is now distinctly raised above the yolk, and the lowermost
layer of cells,
as
a
separate layer,
_
,
arches upwàrd in a grooye below the nerve cord.
:
the downward growth of the muscle ’bundles at the sides
:
this groove is pinched o‘ and becomes the notochord, the
forerunner
of the
vertebral
column.
Underneath
the
Î
notochord the lowermost layer now forms the upper wall
,
:
of the alimentary canal, and the extent to which it remains
in c0ntact With the y01k becomes a distinctiVe mark of the
'
different groups of shes.
'Viewed from the outside the most important signs of
.:
to this stage havebeen the gradual uplifting of the
:
embryo fr0m- the yolk and its
formga
_
:
or shape o_f___ the:future Sh depends greatly
'
thèjembryo1_Ïnow has in which to
_
forms the embryo
round the
_
begin
In the _Hag—shes (Cyclostomes)
’
this position
tightly
yolk_ until”
,
…
:
whilst the head is compressed against the egg-‘
—
capSul£aë(Fig., 13; 2). From this We can understand three [
important features in the adult Hag-sh, the feeble develop—
,
ment of- the brain, the rounded shape and the simple
condition of the gut and kidneÿs. This condition may be,
contrasted With that of the Dog—sh (Scyllium), which has
,.
'
almost the same amount “of yollçÿ'butmore space in its
__
.
_
capsule, _and thus reaches a
of develop:
many—_ ;.Telçîosts _a Similar compression of the
_
Î
headcanbenouœd,espec1allywherethe yolk completely
f
—
llstheegg
—
,
.
îhn'dx thé embfy0
f
): S°,Ônïïbëcpñlssaäuüeÿ£reè-fr9mhefyolk£èXce9t{foi-_thezplacental
“l‘?
adeve10pf * ':‘f
:
,
îhê
!1d—;ÏèXtÏiäl structurés.
'
’
‘
:
Thè'Î'ÿcäung -.
Ï
embryos,
:=. %
'