THEORIES OF REGENERATION 2/3 



that of the larger component. It is possible, I think, to interpret 

 this result in terms of our tension hypothesis. The condition of ten- 

 sion in the larger piece has overcome that of the smaller piece, so 

 that the latter comes to have the same orientation that the larger piece 

 has ; and the development of a head at the free end then takes place. 

 The development of this head holds in check the development of a head 

 at the anterior end of the larger piece in the region of union of the 

 pieces. When two pieces of hydra are united by unlike poles, i.e. so 

 that they have the same orientation, it is found that if the pieces are 

 not too long, a head develops at the free end and none in the region 

 of grafting. The result is similar to that in plants ; the development 

 of the head at the free end suppressing any tendency that may exist 

 to produce a new head by the posterior piece at the place of union. 

 If the pieces united in this way are very long, a head develops at the 

 apical end, and, in some cases, also near the line of union. This may 

 be due to the pieces being so different at the place of union, that 

 a head develops below this region before the unification of the two 

 pieces is brought about, or because the formation of the head at 

 the free end is relatively so far removed from the place of union of 

 the pieces, that it does not influence the development of a head in 

 this region. 



These cases of grafting also illustrate another point of some 

 interest. They show that the development of a head at the anterior 

 end of a piece is not the result of the injury from the cutting or 

 due to the action of some external condition on the free end, for 

 the regeneration may take place when two anterior ends have been 

 perfectly united to each other. The result can only be explained as 

 the outcome of some internal factor such as polarity. 



These examples have been chosen from hydra rather than from 

 tubularia, in which somewhat similar phenomena have been observed, 

 because in hydra the development of heteromorphic structures is of 

 rare occurrence, while in tubularia external influence often calls forth 

 a heteromorphic development. There cannot be much doubt, how- 

 ever, that in tubularia the same kind of internal factors are also at 

 work. 



A more striking illustration of the possible influence of tension of 

 the parts is shown by an experiment with planarians. If the head 

 of a planarian is cut off and the posterior piece is split partially in 

 two along the middle line, as shown in Fig. 31, A, and then one of 

 the halves is cut off just anterior to the end of the longitudinal cut, 

 the result is as follows : A new head develops at the anterior end 

 of the long half (Fig. 31, B\ but no head develops on the posterior 

 cut-surface, provided this part has reunited along the middle line 

 with the long half, and a line of new tissue connects the anterior 



