66
ŸTHE BIOLOGY OF FISHES
development is changed.
’
Hence the inherited environment
is also of fundamental importance.
Regarding the chemical changes that go on within the
egg, We know very little.
When” the egg is laid free in the
water, the shell at once becomes
_
hard; This hardened
chorion, whichis absent in the eggs developing internally,
serves as a protection to the embryo ; but in other ways it is
a disadvantage, since it restricts the growth of the embryo.
It has been observed by Ehrenbaum, however, that the egg—
shell becomes thinner as development proceeds and the egg
'
then increases slightly in “size. Probably, as in the case of
crabs and 10bsters, part of the substance of the shell is With—
;;
drawn and absorbed by the embryo, and this
to a greater extent in the demersal
is more adVanced on hatching, thanin”
'
?
Whilst the embrÿo- seems to béîirhmühe to the chemical
’
Î
inuences, thus tothe' sahmty,1t is very-Sensitive to changes
of temperature;
were placed» in;àtem— ,
of = abouti
the fry hatched out
in temperatures about 4° C. the period has
'f0rty’dàÿs.‘ Observations on other species
:
.
same result,
can
be no doubt
l
that ' under“n0rmal Circumstances this speeding up of the
‘
_
metabolism within the egg is one _of the main factor—S in '
î
causing variations.
_
'
—
…
'
,,
…
.
-
îï_; The period of
“;?
î alsotoberegulatedhythe 1nher1tedconst1tuüon1nthevwde
à
Elasmobran6h8takeonlyafewmonths to develop,— bu—t
‘
Î3k6_$
Among Îthe Teleost‘s, the . Ï-Ï
!
demersal eggs usually
time than the
'
3
_
,
takes =a shért=me: The
';{gç
"
esg; the ÿOmgîîStiblè-' "ll-È
Î_‘_
to
The
”Only
35.5 ï:;;fîâb0üt
fWO‘
.
î;
How these dieféncéS‘ have zà'rise‘ñ,’ We bah
;f:__giægf:'gighut{.fainyimagine,