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THE DEVELOPMENT OF FISHES
65
60,000.
In this and many other— ways it occupieS' an
intermediate position between the freshwater and the pelägi0
and.demersal marine shes.
-
f '_ = _=
,
When the eggs are ripe or mature, asmallopehing appears
‘
in” the shell, the micropyle, throughwhiçh the polar bodies
escape andwater enters.
In this
pressure within
is regulated to the præsurewithout, and'inmany'cfæises the
outer membrane Swells
sometimes increasing the. size
of the egg by as much as four times (Shad and Long-rough
dab).
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A perivitelline space is then formed in which the
embryo is bathed during- its development.
eggs of
’
quite a number of shes, particularly of those living in deep
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water,display the same phenomefion. In:the Herring,on
the other hand, the reverse condition prevails_;
seems to be inelastic and does not expand.
the
yolîk contracts or is compressed and the small perivitelline
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spaceis formed in this way. In most cases the water mixes
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directly'with the yolk atthe time,- and, causes the— egg _to
be00me transparent;
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.
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phenomen0n is due to the fact, that »theÏ-pressureÎä‘7
of the blood and body uids (concentration of salts), in the
Teleosts is much “lower than that of the surrounding. Water-.
But the strange thing is, that afterthe egg has been fertilised
by the'spermat0zoon andthe
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exchange takes place. Gases_ are
to pass ;thrOugh,
‘
oxygen inward‘s and carbonio acid outwàrds,_ but When living
the membranesare impermeable' to Water
its dissolved
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contents,
"Dakin has’ found that the developing eggs
come to have the saine specific density or partial pressure
of " Salts as the parents, though they begin With the
‘
PressureaS the environment.
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It is thus not only the"=substance but
_
passed on
Ï
Ï
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_…_0sËng; ,. As
a‘-.new
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‘
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Afin _—Ç0nnection withthe constitutionalinheritanC€Further,
,
‘
,.;‘x-ê'içfèrhcenvironment
0rmagœglum
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!
embraÿoêisî‘lsaèèustomed— a,raé,ià1ter”edandithewhole
ef .