SUMMARY: THE PROBLEM OF DIFFERENTIAL GROWTH AND EVOLUTION IN 



FISH SKULLS 



The Role of Development 



In the development of the typical teleost skull the eyes and brain become very large 

 at an early stage of development, so that in the lateral aspect the tissues that give rise to 

 the palatoquadrate bar, the suspensorium, the suborbitals, preopercular and opercular series 

 are squeezed between the enlarged eye and the yolk. Hence at their first appearance these 

 structures are arranged around the eye as a center. The mouth is at first extremely small, 

 but by the time of the disappearance of the yolk it is provided with small jaws so that the 

 young larva may catch minute organisms. As development proceeds the typical fish 

 increases rapidly in size, its mouth and jaws grow larger and it feeds on larger and larger 

 prey. Meanwhile the fore parts of tha head are released from the restraining membranes 

 of the egg stage and grow forward, while the body grows backward. Thus the preopercular 

 and opercular series, which arose so close beneath the eye, move backward from it. But 

 while the eye decreases in relative size and the preopercular and opercular series increase, 

 the latter still retain more or less of their circumorbital arrangement, even in the adult 

 stages of typical teleosts. Hence the retention, to a greater or less degree, of embryonic and 

 cetal characters in the adult stage explains a number of characters of certain adult teleost skulls. 



The Three Main Skull Types: Long, High, Broad 



The late embryonic and larval stages of typical teleosts have swollen brains and very 

 shallow bodies, so that the future occipital roof is higher than the back. In long-bodied 

 teleosts, such as the eels, the number of vertebral segments in the adult is very large and 

 there has been very little vertical growth. The neurocranium too remains vertically shal- 

 low and becomes long and narrow. In deep compressed teleosts, such as the porgy or other 

 sparids, on the contrary, the number of segments remains small and as maturity approaches 

 the apex of the back becomes very high, so that it far overtops the occiput. The latter 

 meanwhile has been increasing in vertical depth only at lesser rates than that of the apex, 

 so that the roof of the skull finally slopes sharply upward, while the throat has been extend- 

 ing downward so as to slope toward the rapidly descending gasterion. This great deepening 

 of the body as a whole appears to be associated with the rapid vertical growth of the 

 myomeres. In a third type of teleost, such as Lophius, with wide depressed body, growth 

 in length and height is overshadowed by growth in the transverse planes, so that the mouth 

 becomes very broad, the skull roof broad, flat and low. Thus in fishes no less than in 

 crystals there are primary axes around which tfie characteristic body-form develops. 



Correlation of Skull-form and Body-form 



Obviously in these and hundreds of intermediate cases the skull tends to conform to the 

 shape of the body of which it is a part; its rates of growth in the longitudinal, vertical or 

 transverse planes are correlated to a greater or less extent with those of the body, so that the 

 ratio of, for instance, head height (occipital crest to isthmus) to total body length will vary 



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