720 PATTERNS AND PROBLEMS OF DEVELOPMENT 



which become articulated to the shoulder girdle of the host induce hyper- 

 trophy of the latter. Also, the A tigrinum limb on A. pundatum usually 

 induces hyperplasia, and the reciprocal transplantation, hypoplasia of 

 the spinal ganglia, innervating the limb (Schwind, 1931, 1932). Hetero- 

 plastic grafts of heart primordia between the same two species give hearts 

 functionally donor but with growth rate altered toward that of the host 

 (Copenhaver, 1939). AmUystoma tigrinum somites grafted in place of 

 A . punctaturn somites become much larger than those of the host (Det- 

 wiler, 1938). 



According to these data, species-specific growth rates and amounts show 

 more or less tendency to persist with heteroplastic grafting but can be 

 altered by relations to the host. That different basal metabolic rates of 

 different species and stages may be factors in the experimental results 

 seems not improbable. The "nutritive level" may be largely metabolic 

 level. Amhlystoma tigrinum eyes and limbs on A. punctatum under ordi- 

 nary nutritive conditions grow even more rapidly than in the donor spe- 

 cies, perhaps because with the low metabolism of the host under sub- 

 maximal feeding they are able, in consequence of their high metabolism, 

 to obtain even a larger relative amount of available nutrition than under 

 normal conditions. Older A. punctatum eyes transplanted to younger 

 A. tigrinum are retarded, perhaps because of their low metabolism, com- 

 pared with that of the host. 



Final size of the individual and of its parts is dependent not only on 

 intrinsic growth rate but on initial size and length of growth period, and 

 also, of course, on amount and adequacy of nutritive material available 

 and on other environmental conditions. Primordia of various organs, 

 eye, limb, balancer, etc., in A. tigrinum are initially formed on a larger 

 scale than those of A. punctatum. In "normal" environment most animal 

 species attain more or less definite and characteristic size and proportion 

 because both intrinsic genetic factors and environmental conditions do 

 not differ greatly for different individuals. With alteration in organismic 

 or external environment, size and proportions may be altered, as is evi- 

 dent from the above experiments and from the differential modifications 

 of development discussed in chapters v-vii. 



GROWTH GRADIENTS 



Gradients in rate or amount of growth are very generally characteristic 

 of all stages of development. In early stages growth is generally not spa- 

 tially localized by sharply defined boundaries but decreases radially, 



