CHAPTER III 

 THE INTERNAL FACTORS OF REGENERATION IN ANIMALS 



THE comparatively few cases in animals in which regeneration has 

 been shown to be influenced by external factors have been given in 

 the preceding chapter. In all other cases that are known the factors 

 are internal. By this is meant that we cannot trace any direct 

 connection between the result and any of the known external agents 

 that have been shown in other cases to have an influence on regener- 

 ation. Certain external conditions must, of course, be present, such 

 as a supply of oxygen, a certain temperature, moisture in some cases, 

 etc., in order that the process may go on, but they are without 

 influence on the kind of regeneration, and are necessary for all parts 

 alike. 



POLARITY AND HETEROMORPHOSIS 



Trembley, Spallanzani, and Bonnet knew that, in general, at the 

 end of a piece of an animal from which a head has been cut off a 

 new head develops, and from the posterior cut-surface of a piece a 

 new posterior part is regenerated. Allman was the first to give the 

 name "polarity" to this phenomenon. 1 



In several animals regeneration takes place more readily from one 

 end than from the other of the same cut, and this difference seems 

 to be connected with the kind of new part that is to be regenerated, 

 and not with the actual power of regeneration of the region itself. 

 For instance, if a short piece is cut from the anterior end of an earth- 

 worm, a new anterior end is quickly regenerated from the anterior 

 cut-surface of the posterior piece, but no regeneration takes place, or 

 only after a long time, from the posterior cut-surface of the anterior 

 piece. These relations are reversed if the posterior end of a worm 

 is cut off. There regenerates very quickly a new posterior end from 

 the posterior cut-surface of the anterior piece, but no regeneration 

 takes place, or only after a long time, from the anterior cut-surface of 

 the posterior piece. The new structures that develop after a long 

 time from the posterior surface of a short anterior piece, and from 



1 "There is thus manifested in the formative force of the tubularia-stem a well-marked 

 polarity, which is rendered very apparent if a segment be cut out from the centre of the 

 stem." Allman ('64). 



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