GREGORY: FISH SKULLS 



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only within relatively narrow limits in adults of a given species. The size and shape of 

 every bone in the skull are therefore correlated to a greater or less extent with the general 

 growth tendencies toward excessive length, height or thickness which pervade the body as a 

 whole. In short, there is obviously some sort of regulating mechanism between the growth 

 of any part and that of the body as a whole, such as has already been revealed in other 

 organisms, for example, crabs, dogs and man. Moreover, there must evidently be a large 

 hereditary factor in many of the diagnostic indices of adult head and body length, just as 

 there is in the mammalian skull. But it is equally plain that many proportions change 

 profoundly during individual development, due to the interaction of hereditary with envir- 

 onmental forces. 



Now different rates of growth in either the longitudinal, vertical or transverse planes 

 must apparently be dependent upon one or more of the following: (a) different rates of cell 

 division, (b) a different orientation of the longest diameter of cells to the three primary axes 

 of the dividing egg, (c) some combinations of these two categories, (d) a change in the direc- 

 tion of the axis of most rapid growth. Histological studies of embryonic and larval teleosts, 



Fig. 300. Marked contrast in relative size of head to body. A. CycUptus. B. Hoplopagrus. 



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