REGENERATION 179 



At a somewhat older stage, such experiments have a different 

 result. The parts of the regenerate have then been determined, 

 so that defective limbs are formed after the extirpation of part 

 of the material, or splitting of the bud. 



As regards the problem of how the determination of the 

 various organs within the regenerate takes place, it can be 

 stated on the basis of the experiments already described 

 (p. 176) that this process does not occur in such a way that 

 the determination of each tissue of the regenerate is governed 

 by the cells of similar tissues in the stump. The subdivision 

 of the regenerate into its organs is a process entirely similar 

 to the autonomisation of the organ-fields in embryonic devel- 

 opment. Each part of the regeneration material is as though 

 "imprinted" with a certain determination, depending on its 

 position in the system. The fate of each element is a function of 

 its position. Here again we are dealing with an organisation-field. 



If we want to solve the problem of the origin of this field, 

 we must realise that the regenerate replaces the part that has 

 been removed by amputation, i.e. that its development varies 

 in accordance with the position of the cut. Therefore, the field 

 governing the development of the regenerate must also vary 

 from case to case. The simplest explanation seems to be that, 

 e.g. in the limb of the adult animal, the original limb-field is 

 retained intact. After amputation, it is still present in the 

 stump. From there, it extends into the indifferent material of 

 the regeneration bud, and determines the differentiation of 

 this material. The field in the regenerate, therefore, would be 

 the immediate continuation of the field in the stump, and this 

 would explain the complete harmony between the regenerated 

 organs and those in the stump. 



Certain experimental results, however, conflict with this 

 explanation. We have seen that an early regeneration cone of 

 a fore-limb, grafted onto the stump of a hind-limb, differentiates 

 into a hind-limb, in accordance with its new environment. If 

 however a thin slice of the old tissue of the fore-limb is trans- 

 planted along with the regenerate, the latter will develop into 

 a fore-limb (Milojevic, 1924), (Fig. 64). The disc of fore-limb 

 tissue, therefore, prevents the influence of the hind-limb stump 



