EXTERNAL CONDITIONS 197 



action. On this, however, only further experiment can decide ; 

 but the results are clearly of importance in that they show 

 experimentally that, as we have deduced from mere measure- 

 ments, rapidly-growing organs may exert an influence upon 

 the growth of neighbouring organs. 



§ 7. The Influence of External Conditions 



We next have to consider the effect upon growth-gradients 

 and heterogenic organs in general of the natural or experimental 

 alteration of conditions. 



The heteroplastic experiments with Ambly stoma eyes which 

 we have just been discussing provide us with some facts bear- 

 ing on this problem. In Chapter II reference was made to 

 Twitty's experiments in grafting on to punctatum host-larvae, 

 tigrinum eyes of the same size but less physiological age than 

 the host's eyes. 1 He found that when the hosts were starved, 

 their body-length (five animals, over an average of thirty-five 

 days) decreased by 18 per cent., their own eyes remained un- 

 changed in diameter, and the grafted eyes increased very 

 slightly, by 2-5 per cent. When, however, they were fed just 

 more than enough to maintain their size, their body-length 

 (four animals over an average of thirty-one days) increased 

 by 7 per cent., their own eyes by practically the same amount 

 (6-5 per cent.), but the grafted eyes by 40-5 per cent. One 

 of these specimens showed an increase of 50 per cent, in the 

 diameter of the grafted eye, but without any change in body- 

 length. It is thus clear that competition for food plays an 

 important part in the regulation of relative size of parts. The 

 rapidly growing eye, whether rapidly growing by virtue of its 

 youth or its higher specific growth-intensity, can draw dis- 

 proportionately upon the supply of food available (for further 

 discussion of this general problem, see Jackson, 1925, and 

 Huxley, 1921). It is clear that much valuable work would 

 be possible in determining the quantitative growth-relations, 

 in varying nutritive conditions, of heteroplastic grafts of the 

 same and of different ages, and of homoplastic grafts of 

 different age from their host. (Fig. 31.) 



We have already considered the effect of poor nutritive 

 conditions upon the forceps of male Forficula (Chapter II) and 

 upon the dorsal crest of male newts (Chapter VI). Perkins 

 (1929) has analysed the effect of the parasite Sacculina upon 



1 A similar regulation of rate of development was found by Choi 

 (193 1 ), who grafted lateral half -larvae of two species of frog together. 



