Studies on Regulation. 273 



new tissue grows out perpendicularly to the cut surface, /. e., at 

 right -angles to the longitudinal axis of the body when the parts 

 are in their usual position (Fig. 28). In my first paper on Lepto- 

 plana (Ghild, '04a) I showed that the direction of outgrowth of 

 new tissue was determined, at least in part, by the direction of the 

 mechanical tension to which it was subjected. As regards the 

 present case it was pointed out above that when these outgrowths 

 of new tissue are subjected to tension it is approximately perpen- 

 dicular to the cut surface. Thus, in this case the direction of the 

 outgrowth is determined by the direction of the tension to which it 

 is subjected. If the specimen were capable of holding the two 

 ends in the position indicated in Fig. 29 and if the new tissue were 

 much used for attachment there is little doubt that each outo-row^th 

 would soon become oblique with respect to the surface from which 

 it arose and approximately parallel to the longitudinal axis of the 

 body. But in the case under consideration the functional sub- 

 stitution of the margins of the old part for the tail became more 

 and more complete as time went on. The animal seemed to 

 become more and more accustomed to the altered positions of 

 parts and coordination apparently became more perfect. The 

 form and relative size of the new parts did not change appreciably 

 from the condition represented in Fig. 28. 



This case, like others described in previous papers, indicates 

 how readily the course of regulation may be altered when the 

 really essential conditions are changed. It also shows very clearly 

 that the power of a piece to attain the characteristic form of the 

 species is dependent upon characteristic functional activity. 



Repeated attempts to obtain other specimens of the same sort 

 were unsuccessful. Usually the contraction of the cut surfaces 

 was not sufficient to bring the transverse surface into line with the 

 longitudinal. 



3. Duplication of the Posterior End by Protrusion of the Pharynx 

 from the Cut Surface. 



Like the preceding, this case is interesting, not merely as an 

 abnormality, but as adding to the evidence regarding the factors 

 concerned in the growth of new parts. 



The individual, a worm of average size, was cut transversely 

 through the anterior end of the pharynx leaving only a small part 



