134 



PROBLEMS OF RELATIVE GROWTH 



160 180 EOOgm 



X 

 75 

 70 



60 



50 

 40 

 30 



120 140 IbO ISO 200jm 



100 120 140 160 IBO ZOOjm 



D 

 Fig. 70. — Changes in relative weight of vari- 

 ous organs of the cat during pre-natal life. 

 (A) Head ; (B) trunk ; (C) fore-limbs ; (D) hind- 

 limbs ; all against total weight. 



the same time, and in 

 such a way that, as Sey- 

 mour Sewell, says (1. c., 

 p. 9), ' when a segment 

 of the body divides into 

 two, as for example in 

 the development of the 

 abdomen, the total pro- 

 portional length of the 

 two daughter-segments 

 is always greater than 

 the proportional length 

 of the parent-segment '. 

 As result, the propor- 

 tional length of the ab- 

 domen increases steadily. 

 But this change in rela- 

 tive size is quite distinct 

 in character from the 

 change in proportional 

 size in different regions 

 of the antennae of the 

 same animals (p. 85), 

 in which the definitive 

 number of segments has 

 been differentiated before 

 the growth-changes oc- 

 cur. Many ontogenies 

 would undoubtedly yield 

 interesting results if ana- 

 lysed quantitatively in 

 the light of the principle 

 of heterogony on the one 

 hand and of that of 

 antero-posterior develop- 

 ment on the other. This 

 would apply especially 

 to ontogenies of primi- 

 tive type showing con- 

 tinuous gradual change, 

 such as those of Trilo- 

 bites (cf. Raw, 1927). 

 It is possible that the 



