CHAPTER V 



THE RECONSTITUTION OF ISOLATED PIECES IN RELATION TO 

 REJUVENESCENCE IN PLANARIA AND OTHER FORMS 



THE RECONSTITUTION OF PIECES IN Planar ia 



In consequence of the ability of isolated pieces cut from the body 

 to develop into complete individuals, the various species of Planaria 

 have served to a very large extent as material for the study of 

 "form-regulation," "regeneration," "restitution," as the changes 

 which occur in such pieces have been variously called. The mor- 

 phological and histological features of the reconstitution of such 

 pieces into new wholes have been repeatedly discussed by various 

 authors and for various species. Since the essential features of 

 the process do not differ widely in the different species, a brief 

 description of reconstitution as it occurs in P. dorotocephala will 

 serve the present purpose. The reconstitution of such a piece as 

 a in Fig. 14 is shown in Figs. 15-17. The cut surfaces of the piec 

 contract after its isolation, and in the course of two or three days 

 outgrowths of new embryonic tissue appear on these surfaces, 

 these outgrowths being readily distinguishable from other parts of 

 the piece by the absence of the dark-brown pigment characteristic 

 of the species. In Fig. 15 and following figures these outgrowths 

 of new tissue are marked off from other parts by lines which indicate 

 the boundaries between new and old tissue. During the next two 

 or three days the anterior outgrowth develops into a head with 

 eyes, cephalic lobes and, as the section shows, a new cephalic 

 ganglion, and the posterior outgrowth develops into a posterior 

 end (Fig. 16). At about the same time the new pharynx becomes 

 visible, near the posterior end of the old tissue of the piece, and the 

 intestinal branches present in the piece begin the changes which 

 end in the formation of an alimentary tract like that of a whole 

 animal. The developing animal also elongates and decreases in 

 width, the postpharyngeal region grows at the expense of the pre- 

 pharyngeal, and finally an individual results (Fig. 17) which is in 

 all respects, so far as can be determined, a whole animal of small 



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