THE FUNCTIONAL PERIOD OF DEVELOPMENT 421 



§2 



Growth is also directly responsible for a certain type of further 

 differentiation, namely, change of proportions. There are at least 

 five factors involved here. One concerns the specific growth in- 



Fig. 203 

 Inherent growth-rates in Hmb-rudiments. Left, a larva of Amblystoma punctatum 

 and right, one of Amblystoma tigrinum between which the left fore-limb rudi- 

 ments were exchanged at the tail-bud stage: 50 days after operation, after 

 maximal feeding of the larvae. The grafted limbs {gr.) are approximately of the 

 same size as the corresponding unoperated limbs of the donors. (Redrawn from 

 photograph in Twitty and Schwind, ^owrn. Exp. Zool. lix, 193 i.) 



tensities of the organs, which will determine the main features of 

 the growth-equilibrium between them and the body : this has been 

 dealt with in Chap, x (fig. 203). The second concerns growth- 

 gradients, which will influence the growth of parts within single 



