80 HOW ANIMALS DEVELOP 



develop in one way. It means that if the lump of 

 presumptive leg tissue develops at all, it will develop 

 into a leg. But within the lump a good deal of 

 changing about of various parts can go on without 

 affecting the result. If a piece is removed the leg- 

 rudiment will nevertheless be able to form a com- 

 plete leg. A system like this, where the various parts 

 can be changed round or taken away without 

 affecting the result of the subsequent development, is 

 called by the grand name of a harmonious equipotential 

 system. As a matter of fact, it is very doubtful if any 

 completely harmonious equi-potential systems exist. 

 There are nearly always some differences between the 

 different parts of the system so that changes of some 

 kind have no effect but others have. The leg-rudiment 

 may have a tendency to form a complete leg out of 

 whatever tissue is available, but the tissue is never 

 quite labile in every way and therefore, after some 

 injuries, cannot be moulded into an entire limb. But 

 the tendency to form a whole leg out of an injured 

 rudiment suggests that the tissue is, in some way 

 which we do not yet understand, in equilibrium 

 when it forms a complete whole. Dragomirov has 

 recently studied the question in the eyes of the newt 

 embryo. He cut out and isolated various parts of the 

 eye rudiment and watched how they returned to the 

 normal shape. He found that the isolated pieces could 

 develop in various ways, which represented different 

 short-cuts, as it were, back to the equilibrium position 

 of being a complete eye. 

 Once, when I was working in Germany, I met a 



