DYNAMICS OF REGENERATIVE PROCESSES 



221 



Giard and Caullery, as well as the experiments of Driesch, resemble 

 those in Campanularia, and Driesch also expresses the opinion that 

 this is a case of reversibility of the processes of development. 



The idea that the process of development is in certain forms reversi- 

 ble is also supported by the experiments of Frank Lillie on fresh-water 

 Planarians. Lillie found that adult fresh- 

 water Planarians if exposed to starvation 

 not only become gradually smaller in size 

 but ultimately return to an embryonic form ! 

 These experiments have been repeated and 

 confirmed by Schultze. 



There is a possibility that a definite kind 

 of chemical substances must be present in 

 order to make development, regeneration, 

 and growth possible. Such tissues as contain 

 these substances (or mixture of substances) 

 may be called embryonic. If this idea be 

 correct, and if it be true that phenomena of 

 development are reversible, to a great 

 extent in a few forms, and to some extent 

 perhaps generally, --the question might be 

 raised whether or not one of the conditions 

 of regeneration is the transformation of adult 

 tissue into more embryonic tissue. If this 

 were true, the power of regeneration of an 

 organism might depend upon the degree 

 of reversibility of the processes of development in such a form. It is 

 certainly in harmony with such an idea that forms like Hydroids, 

 Ascidians, and fresh-water Planarians, where the reversibility of the 

 process of development is most outspoken, possess also the greatest 

 power of regeneration among animals. The idea suggested is further 

 supported by the fact that the power of regeneration by the embryo is 

 often considerably greater than the power of regeneration of the same 

 form in the adult stage. The tadpole of a frog is capable of regenerat- 

 ing a leg, while this is impossible in the adult frog. 



This removes the contradictions into which we fall if we try to rep- 

 resent the power of regeneration as parallel to the position of an animal 

 in the natural system. We find Annelids, such as the leech, whose power 

 of regeneration is decidedly less than that of some vertebrates, e.g. the 

 salamander. If we cut off the tail of the salamander, a complete regen- 

 eration of this organ with all its parts, bones as well as spinal cord, 

 occurs. If we cut off a number of segments from a leech, the wound is 



FIG. 64. 



