Weismann's Explanation of Regeneration 143 



other for that of the tail end, and that one or the other 

 becomes active according as the stimulus due to lying un- 

 covered, is applied to the anterior or posterior surface of 

 the cell concerned." m 



Finally according to epigenetic theories the regenera- 

 tion of the hydra is a process which does not differ es- 

 sentially from any other process of regeneration, but ac- 

 cording to Weismann's theory the following complicated 

 additional explanation becomes necessary. 



"If one divides a hydra in a longitudinal plane the 

 two halves grow again into entire individuals, irrespective 

 of the plane of section. As a transverse section of the 

 animal at any point which may be selected is followed 

 likewise by the complete reconstruction of each of the 

 two halves it follows that every part of the body of the 

 hydra must be capable of regeneration in a threefold 

 direction, namely in the three directions of space. As the 

 body is differently constructed in these three directions 

 we are forced to the conclusion that each of its cells must 

 contain groups of determinants of three different kinds. 

 * * * And it cannot be the quality, but the direction 

 from whicfi the stimulus of the wound comes to each 

 cell, which will decide for it which of the three groups of 

 determinants will become active/' 118 



We believe that we do not pronounce too severe a 

 judgment, if we affirm that so artificial a hypothesis 

 demonstrates the absolute incompetency of preformation 

 theories to explain the phenomena of regeneration. 



What is the conclusion which can be drawn from all 

 that we have said thus far in the present chapter? 



11T Weismann: Das Keimplasma. P. 169. 

 ll8 Weismann : Das Keimplasma. P. 170. 



