226 KDMl'NI) I!. WILSON. 



the remarkable quickness of the process is doubtless due to the 

 extremely plastic character of the tissues in the animal, ordinarily 

 shown in its frequent and very marked changes of form. In 

 planarians the initial remoulding takes place much more slowly 

 and forms an initiative for other regulative changes by which the 

 piece is permanently remodelled into a new form. In Rcnilla 

 such regulative changes are apparently absent, or present in 

 only s nail degree, and the ensuing process of neomorphosis 

 tends to counteract the initial rnorphallaxis and restore the orig- 

 inal form. Morgan * has shown that even two species of the 

 same genus may differ materially in the ratio between remould- 

 ing of the old tissues and the formation of new, the latter process 

 being more extensive in P. lugnbris than in P. inacnlata. Rcnilla 

 appears to offer a case in which both processes occur, but the 

 former produces a result that is only temporary, owing to a 

 high degree of specification in the members of the colony. That 

 this specification is, however, not absolutely fixed is evident from 

 the fact that heteromorphosis may occur, as shown in the forma- 

 tion of a new peduncle from a lateral group of polyps (Torrey), 

 or in the development of a polyp instead of a peduncle from a 

 severed axial polyp ; and the same is proved by the establishment 

 of a new colony after removal of the budding zone, as recorded 



above. 



7(><">i,'>i;i<-.\i. LABORATORY OF COLUMBIA UNIVERSITY, 

 January 15, 1903. 



1 T. H. Morgan, " Growth and Regeneration in Planaria luguliris,'' Ai ch. 

 XIII., i, 2, 1901. 



