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298 THEORETICAL BIOLOGY 



quantity of material left over from furnishing parts of the 

 framework in their normal size ; for the circulating stream 

 of fluids enables them to make up the lacking substance. 



We have got the fundamental information about regenera- 

 tion from experiments on earthworms and planarians. Here 

 it appears that each transverse slice cut out of the animal 

 at some selected point is able to remake the whole, right 

 forward to the anterior end and right back to the posterior. 

 In this we perceive clearly the action of an internal equilibrium 

 extending throughout the entire animal. 



In these animals the material for the reconstruction of 

 > the equilibrium lies everywhere at the disposal of the impulse- 

 systems. An exception is offered only by the head of 

 planarians. If this is cut through transversely, each surface 

 of the wound is capable of regenerating only the anterior half. 

 The consequence is that only the posterior portion of the 

 head regenerates a normal head : the anterior portion dupli- 

 cates itself, and produces a Janus-head incapable of living. 



If we cut a planarian lengthwise, each half regenerates 

 the missing side, and we get two normal planarians. If, 

 beginning at the anterior end, we carry the cut only through 

 the first half of the animal, the edges of the wound gape apart 

 on account of the pull of the lateral muscles, and each half 

 regenerates the missing side. The result is a two-headed 

 planarian. 



This experiment (which, after what has been said, holds 

 nothing surprising) has played a fateful part in the history 

 of biology, for from it Vulpian concluded that there could 

 not be any vital force, since vital force would never create 

 monsters. In the state of biological thought at that time, 

 Vulpian could not conceive of a vital force as other than one 

 endowed with reason. From his point of view, there must 

 either be a human-like director in every animal, or else none 

 at all. We have become more cautious, and we speak only 



