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.” 117 
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 which the stimulus of the wound comes to each 
cell, which will decide for it which of the three groups of 
determinants will become active.” 1*§ 
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? 
117Weismann: Das Keimplasma. P. 169. 
118Weismann: Das Keimplasma. P. 170. 
