122 



GROWTH 



from the head toward the tail region, in accordance with the 

 law of developmental direction. 



777. Attainment of Adult Proportions 



In fishes, the observations of Hecht and others indicate that 

 the permanent adult form and proportions are reached rela- 

 tively early. For the dogfish, Hecht 13 found that in specimens 

 40 to no cm. in length the weight forms a constant fraction 

 (.00274) of the cube of the total body length. From this he 



concluded that "there is a 

 constancy of form within the 

 species studied, which is ad- 

 hered to throughout the life of 

 the individual." 



Later, Hecht 14 investigated 

 a series of eleven species 

 (seven families) of bony fishes 

 at different sizes. He found 

 here likewise that, for the 

 sizes observed, there are in 

 each species definite and con- 

 stant ratios between total body 

 length and the lengths of the 

 head, trunk ("body"), and 

 tail, as well as the width and depth of the body. As shown in 

 Figure 66, this means that a small fish (A) has the same body 

 form as a large fish (B), all parts in the latter being enlarged 

 proportionately. The data for one species (Anchovia) are 

 shown in graphic form in Figure 67. The "curves" for the vari- 

 ous dimensions are all of the straight-line type, according to the 

 formula 



Figure 66. Showing the similarity of exter- 

 nal form and proportions in a small fish 

 (A) and a large fish (B). The small circles 

 indicate a linear enlargement of one-half in 

 the length of the body and parts of the 

 larger fish. After Hecht. 14 



where y is the linear dimension of any part, x the corresponding 

 total body length, and c a constant fraction for the given part. 



