REGULATION OF FORM AND STRUCTURES
107
of the skull.
This was the case also in the ancient, Cœla—
canthids (Stensiô, 1921), and possibly Crossoptefygii (Watson,
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1912); In the larval Herr—ing the .palatine appears to separate
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from the side_of the ethmoid cartilage—in
larva,
Then, as mentioned above, it drops down to form
jaW of the. Elas10branchs-ad Ganoidsl:
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however, itremains in touch“withzthe ethmoid
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as a buttress for this above and the suspensorium below.;
in this position -it has considerable inuenceon the Shape0f
the skull. The membrane of the mouth is continued over
this arch wellin:froi1t and in it there forms-the maXillary,
an important membrane bone in'the Teleost—s With posterior
ventral ns (Abdominales), and later, in front of. the
maxi‘llary, the premaXillary. These '
bones naturally are
subject to great variation, like the lower jaw.
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That these changes in the mouth and head have come
from the movements Of the body and the changes in the
alimentàry canal, can hardly be doubted. Thé complica—
tions'are so numer0us, however, that any general description
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of the relations and events cannot be given. Each group of
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shes. has to be
yth_separately and thefew examples
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as anin‘dièation of what has
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.;
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;
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,
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In the course of the many changes gone through by the
bony shes certain critical periode, more critical than others,
have arisen in which it would seem impossible for the in—
dividuals to survive.
These are indicated by various signs,
jaws working on the top of the skull, skull bent downwards
With: mouth and teeth reduced totiny proportions, huge
spinous processes on the head and body, ns enormously
deVeloped ;orfdiscarded—ihdicating the severity:o£1£the
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’
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strugglethe orgànism has— experienced in co-ordinating the
growing structures.
In some cases we can see in detail
how the form£has changed. ‘
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Â
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It has been indicated aboVe,’ thatthe lbalaÏceof shes hàs
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been quite indenite in“ the beginning-f :In'Ïmaking their