68
THE BIOLOGY OF ”FISHES
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form in some important types ; thus in the Lampreys at one
end, Sturgeon and Dipnoi at the other, the segmentation
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is complete but not quite-equal, just as in the frogs. In the
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Teleosts and Elasmobranchs, on the other hand, the greater
portion of the living protoplasm of the egg collects into a
disc at the one end.
There it divides up and only gradually
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manages to develop a layer of cells round the yolk.: This
meroblasi‘ic form of segmentation is undoubtedly the more
specialised. In some Teleosts, however, especially the
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Clupeicls, we nd a supercial segmentati0n of the yolk,
distinct fr0m- that Of the disc, and this is probably due to a
large quantity of the living —protoplaism remaining,
the
:_
substance of the"yolk. The same phenomenon appearsto
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occur
of an ancient
by
’
Bashford Dean. This seems to bean intermediate
etage-betWeen the more and the less complete mastery of
_
the
forrhative protoiolasm.
‘
From the
biological standpoint, we may question
of yolk has been an advantage;
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More”: ïadVanced forms are able to do without it, perhaps by
_
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“improving the quality of the transmitted materials, but :
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by bringing the egg and embryo into a
different kind of environment. Thus, we may connect the
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fact, that the shes have remàined stationary at a, com‘;
paratively low level of vertebrate develOpmet,_with the
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_'
conditions under Which the
to
in: the earlies—t' stages.
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l l _Ï _;
”
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…
zEMBRYO ;;
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.
,
In the beginning. ‘fhèlcléàvageand'
number
.
of
signs of the life within.
We can hardly speak of Ïan embryo until somestructu‘re is
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.
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apparent ;' before that it is simply—referred to
:.
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disc or blastosphere. This works-itself into a gaStrulà, the
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a“
in which the outer layer gradually extends
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".Îover the surface==of the yolk and goes to form the sensor—yetnd
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i
; ‘“ËÎÏÊÎVOÙS _s‘tems,.whil‘st the inner layer ‘pro'ceeds-