178 REGENERATION 



under the influence of this environment. An experiment by 

 Schotte and Hummel (1939) has demonstrated clearly that the 

 tissue of young buds really is of an entirely indifferent, 

 "embryonic" nature. They transplanted young regeneration 

 buds into the inner eye-chamber. Here the buds formed a lens 

 under the influence of the retina. 



Older regeneration buds, on the other hand, have already 

 been determined for development in a definite direction. After 

 transplantation they produce the same material as they would 

 also have formed if they had not been displaced. The moment 

 of determination at which the buds lose their indifferent 

 character depends on various circumstances; on the average, 

 the indifferent stage of limb buds lasts about two weeks. 



Some Russian authors have in later years raised objections 

 to these experiments and their explanation (e.g., Polezajev, 

 Liosner). They pointed out that in many of these experiments 

 the possibility was not excluded that the graft did not go on 

 developing, but that it was repressed and replaced by the 

 stump's own regeneration bud. This would then explain why 

 the properties of the regenerate are in accordance with those 

 of the stump, regardless of the origin of the graft. In reality, 

 early regeneration buds would not be indifferent, but possess 

 a certain degree of labile determination already. In support of 

 this view they described experiments the results of which 

 did not agree with those mentioned above. An early regeneration 

 bud of a tail, for instance, produced a tail-like appendix after 

 transplantation onto an amputated limb. So far, however, their 

 results are not very convincing. 



In regeneration, just as in embryonic development, de- 

 termination seems to take place stepwise. First the regeneration 

 bud as a whole is determined to form a fore-limb, or hind-limb, 

 etc., but the fate of each individual cell is not yet fixed. Schaxel 

 (1922) has shown that part of an already determined regenera- 

 tion bud is still able to produce a harmoniously built limb after 

 transplantation. If a limb regeneration bud is split into two, 

 each half will produce a complete limb (Swett, 1928). On the 

 other hand, two buds which are already determined, may still 

 fuse so that they form a single, harmoniously built limb. 



