126 THE GERM-PLASM 



ivhich are physiologically important and are at the same tivie 

 frequently exposed to loss. In all probability this view is the 

 correct one. 



3. Facultative or Polygenetic Regeneration 



The tail of a lizard or the limb of a Triton grows again when 

 it has been cut off. but the part amputated does not reproduce 

 the entire animal. In some segmented worms, on the other hand, 

 such as Nais and Lmnbriculiis. not only does the amputated tail- 

 end become restored, but this end itself reproduces the anterior 

 part of the body, so that two animals are formed from one. 



This fact evidently cannot be deduced merely from the 

 assumption we have made with regard to supplementary deter- 

 minants : for were this the case, determinants of one kind only 

 — viz., those which are necessary for the construction of the lost 

 part — would be present in the cells. But in the above instances 

 the sa)iie cells give rise to entirely different parts, according to 

 whether they are situated on the surface which is anterior or 

 posterior to the plane of amputation : in the former case they 

 reproduce the tail-end. and in the latter the head-end. The 

 fact that both parts grow again when the worm is cut into two 

 through any region of the body, proves that regeneration in 

 either direction may proceed from the same cells ; it therefore 

 follows that the cells situated in any particular transverse plane 

 of the body are not merely provided with the supplementary 

 determinants for the formation of the head- or tail-end only, but 

 every cell can react in either way. according to whether it is 

 situated anteriorly or posteriorly to this plane. In order there- 

 fore to explain the twofold action of these cells in accordance 

 with our fundamental view, — which presupposes that the cells 

 taking part in regeneration are arranged and controlled by the 

 forces situated within them, and not by an e.xternal agency, — it 

 seems necessary to assume that each cell possesses two different 

 supplementary determinants, one for the construction of the 

 head-end, and one for that of the tail-end ; and that the one or 

 the other becomes active according to whether the stimulus, due 

 to the exposure of the cell, is applied to its anterior or to its 

 posterior surface. 



Before attempting to verify this assumption. I must mention 

 certain cases in which the regenerative activity of the cells may 

 even be threefold. 



