298 PRINCIPLES OF EMBRYOLOGY 



general increasing the growth of the limbs immediately posterior to it 

 and depressing the rate of the Hmbs immediately in front. The mechanism 

 of the effect— whether the fast-growing limb operates by secreting some 

 growth-promoting substance or by competing in some way for a 

 limited supply of available raw materials— is quite unknown. 



One of the most extended series of studies of the physiology of growth 

 rate of individual organs is that by Harrison, Twitty and others on the 

 eyes and other organs of the Mexican salamander Amhly stoma (Reviews: 

 Harrison 1933, Twitty I934, Needham 1942, Reeve and Huxley 1945). 

 The eye of one species, A. tigrinum (the normal axolotl) is very much 

 faster growing than that of the nearly related species A. punctatum. It was 

 first shown that if the eye-cups are interchanged and transplanted from 

 punctatum to tigrinum or vice versa, each type retains its own characteristic 

 growth rate even in the new situation. The growth rates are therefore 

 inherent in the eye-cups themselves. 



When organs are grafted into other animals of the same species but 

 different age, the growth rate of a transplant younger than the host is 

 speeded up, and that of an older one slowed down, until the grafted 

 structures have reached a size appropriate to the body in which they lie. One 

 hypothesis to account for this would be that the young organs have a greater 

 assimilative efficiency than older ones, and are therefore able to obtain 

 more than their normal share of nutrients from the blood stream when 

 they have only older organs to compete with. But that can hardly be the 

 whole story. Even when the competitive demand for nutrients was m- 

 creased as much as possible (by starvation, to such a point that the host 

 body lost weight, together with amputation of the tail which caused this 

 to regenerate) the young transplanted eyes were still able to grow. 

 Twitty came to the conclusion that at least two factors must be involved; 

 not only a decreasing assimilative efficiency of an organ as it ages, but 

 also an increase with age of the richness of the nutritive suppHes in 

 the blood (Twitty and Wagtendonk 1940). These ideas and observa- 

 tions are obviously closely related to those of Hammond mentioned 



above. 



In other experiments by Twitty, the two main elements of the eye— 

 the eye-cup and the lens— were combined in different ways. Here there 

 was definite evidence of the influence of the growth rate of one element 

 on that of the other. For instance, if a large eye-cup of Amhlystoma 

 tigrinum is combined with the small lens of ^. punctatum or is allowed to 

 induce a lens in punctatum skin, we find that in the compound eye, the 

 eye-cup of tigrinum grows more slowly than usual while the lens of 

 punctatum grows more rapidly, so that the two finally come to bear a 



