THE LATER STAGES OF DEVELOPMENT 



155 



Though growth is highly dependent upon such external 

 factors as temperature and food supply, yet each species has 

 its own rate of growth. This was clearly shown by experiments 

 in which the primordia of a limb were exchanged between 



embryos of two newt species, 

 viz. Amblystoma punctatum 

 and A. tigrinum, which at their 

 later stages show different 

 growth rates. It was found that 

 the grafted limbs retained their 

 own growth rates, and did not 

 adapt themselves to the slower 

 or faster rate of the host 

 (Harrison, 1924), (Fig. 55). 



Moreover, the various organs 

 also have their own character- 

 istic growth rates. This causes 

 changes in the body proportions 

 in the course of development. 



Finally, various organs in- 

 fluence the growth of other 

 organs. This was demonstrated 

 by combining, by means of a 

 heteroplastic transplantation, a 

 lens of Amblystoma punctatum 

 with an eye-cup of A. tigrinum. 

 The more rapidly growing ti- 

 grinum eye-cup stimulated the 

 more slowly developing punc- 



Fig. 55. Larvae of (a) : Am- 

 blystoma punctatum and (b): 

 A. triginum, between which 

 the left fore-limb primordia 

 have been exchanged at an 

 early stage. After 50 days, the 

 transplanted limbs are about 

 the same size as those of the 

 animal from which they origin- 

 ate. After Huxley and de Beer. 



tatum-lens, to faster growth. 

 The same is true of a tigrinum lens when combined with a 

 punctatum eye-cup. A mutual adaptation in size and growth 

 rate occurs between the two parts (Harrison, 1929; Twitty and 

 Schwind, 1931). In Rotmann's experiments, too, the originally 

 maladjusted eye-cup and lens of the heteroplastic combinations 

 (see above p. 133) were later on found to show mutual 

 adaptation in size by differential growth. 



This mutual influence is also clearly expressed in the central 



