FORM AND GROWTH 



123 



Hecht recognized that this rule does not apply to the viscera, 

 whose varying growth rates in the dogfish had been shown by 

 Kellicott. 29 Kearney's 27 data also show that the constant relation 

 between body length and body weight does not hold for dogfish 

 less than 30 cm. long, which would probably correspond to 

 a year of age (Kellicott). Hecht furthermore noted that the 

 early attainment of a constant external form occurs only in 

 vertebrates with indeterminate 



4 5 6 7 8 9 10 11 12 13 J4 

 Total Lcnf tfi - Cm. 



Figure 67. Graph showing the relations be- 

 tween total length and various linear di- 

 mensions in the fish, Anchovia brownii. 

 The individual measurements, grouped 

 closely along the straight lines, according 

 to the formula: y (length of part) = c 

 (constant fraction) times x (total body 

 length). After Hecht. 11 



growth, i.e., with no fixed 

 limit of adult size (as is said to 

 occur in fishes and amphibia) ; 

 "whereas in animals having 

 determinate growth, the ex- 

 ternal form changes continu- 

 ally during the period of 

 growth, and as soon as the 

 form becomes constant, growth 

 ceases." He also concluded 

 that this early constancy of 

 form in the fishes is an adapta- 

 tion to aquatic life. 



Apparently but few measurements have been made showing 

 changes in the external form of amphibia. Donaldson and 

 Schoemaker 11 observed that in frogs from one year of age to 

 adult the leg-bone lengths bear a nearly constant ratio to the 

 body length. This would imply constancy of form at least in 

 this one feature. 



We may compare the growth in external dimensions of the 

 fish with the growth found by Calkins and Scammon 6 in the hu- 

 man fetus (Figure 68). They constructed graphs to show the 

 relations of total body length to about 70 linear dimensions of 

 the fetal body. Nearly all these present a straight-line relation- 

 ship, as shown by four typical cases in Figure 68. The 70 meas- 

 urements may be grouped into three general classes. The first 

 group includes the thoracic dimensions (exemplified by chest 



