388 



T. H. Morgan. 



have found that none such is present as a rule. In the few cases 

 in which a pharynx appeared in the middle of the piece, the piece 

 may have come from the region near to or through the pharynx- 

 chamber of the old worm. 



Turning now to the results of regeneration of very short cross- 

 pieces of Planaria stmplicissima, it was found that double headed 

 pieces are sometimes obtained from the more anterior regions, 

 Fig. II, as in P. ?naculata, Figs. 2-5. When short cross-pieces 



of P. simplicissima are cut off posterior to the pharynx-chamber a 

 number of them produce a head at the anterior end and a tail at 

 the posterior end, especially if they are rather long. Fig. 15; but 

 short pieces and sometimes some of the longer pieces also produce 

 quite often a pointed structure at the anterior end. In the ma- 

 jority of cases these anterior structures never develop into any- 

 thing different, and resemble a tail in all respects. In a few cases, 

 however, the pointed structure may become a head after some 



