SEGMENTATION 77 



very curious lights on the nature of living things. To an under- 

 standing of the nature of the mechanics of living matter and its 

 relation to matter at large they offer the most hopeful line of 

 approach. I allude especially to the examples in which it has 

 been established that the part which is produced after mutilation 

 is a structure different from that which was removed. The 

 term "regeneration" was introduced before such phenomena 

 were discovered, and though every one recognizes its inapplica- 

 bility to these remarkable cases, the word still misleads us by 

 presenting a wrong picture to the mind. The expression " hetero- 

 morphosis" (Loeb) has been appropriately applied to various 

 phenomena of this kind, and Morgan has given the name "mor- 

 phallaxis" to another group of cases in which the renewal occurs 

 by the transformation of a previously existing part. 10 But we 

 must continually remember that all these occurrences which we 

 know only as abnormalities and curiosities must in reality be 

 exemplifications of the normal mechanics of division and growth. 

 The conditions needed to call them forth are abnormal, but the 

 responses which the system makes are evidences of its normal 

 constitution. When therefore, for example, the posterior end 

 of a worm produces a reversed tail from its cut end we have a 

 proof that there must be in the normal body forces ready to 

 cause this outgrowth. The new structure is not an ill-shaped 

 head-end, for, as Morgan shows, the nephridial ducts have their 

 funnels perforating the segments in a reversed direction. The 

 "tension" of growth is actually reversed. 11 So also when in a 

 Planarian amputation of the body immediately behind the head 

 leads to the formation of a new reversed head at the back of 

 the normal head, while amputation further back leads to the 

 regeneration of a new tail, these responses give indications 

 of forces normally present in the body of the Planarian. Such 

 facts open up a great field of speculation and research. Espe- 

 cially important it would be to determine where the critical 

 region may be at which the one response is replaced by the 



10 Morgan, T. H., Regeneration, 1901. 



11 It would be interesting to know whether growth continues at the original 

 posterior end after the new "posterior" end has been formed in front. 



