FIELDS AND GRADIENTS 273 



The results of the regeneration experiments, as well as those con- 

 ducted on embryos undergoing embryonic development, agree in 

 demonstrating two important points. The first is that tissue which 

 is about to differentiate into a given structure is at the outset un- 

 determined, and therefore capable of differentiating into other 

 structures, of wholly different type. The second is that the actual 

 decision as to the fate of such undetermined tissue rests with its 

 position relative to some major system. In the amphibian egg, the 

 determining factors are the level of the tissue along the main egg- 

 axis, and its distance from the organiser region (see p. 139)- In the 



Fig. 128 



Lack of determination in early regeneration-buds. The smaller limb here shown 

 (right) was produced from the early regeneration-bud of a Triton tail grafted on to 

 the stump of a hind-limb. Left, unoperated hind-limb of other side. (From 

 Wells, Huxley and Wells, The Science of Life, London, 1929, after Weiss.) 



case of the Planarian cut transversely, the determination of the 

 pieces of freshly regenerated tissue are controlled in relation to the 

 polarity of the whole organism : front edges of hind halves produce 

 heads, hind edges of front halves produce tails. In regeneration in 

 newts the type of differentiation is controlled by the local environ- 

 ment of the regeneration-bud; this exerts qualitatively different 

 effects in different regions of the body (e.g. region of leg as against 

 region of tail). The material of the ^arly regeneration-bud is in- 

 different. So far, it has been shown that its capacities of differentia- 

 tion include organs of such different type as limb and tail ; it would 

 be of great interest to determine whether it was so completely 



HEE 18 



