296 Experifnerital Zoology 



plied with a mouth opening. The reversal of polarity described 

 for reversed grafts of hydra has been discovered by Mrs. Mor- 

 gan to take place in planarians also when the piece is very short. 

 If, for instance, anterior cut surfaces of two pieces are united, 

 and one of the pieces is subsequently cut off near the line of 

 union, that piece if very short produces a head at its posterior 

 exposed end. Here also it appears that the results may be due 

 to the influence of the old part. In this case there is no question 

 of absorption of the smaller piece, but there is on the otherhand 

 ample opportunity for inwandering cells to pass from the large 

 part through the small piece into the new material. The result 

 is further complicated by the fact that very short pieces of other 

 species of planarians, that are not grafted, may produce a head 

 at the anterior end and another at the posterior end, and while 

 at present this result has not been obtained with Plagocata, the 

 possibiHty still exists that the results may be due to the cut-off 

 pieces that have been tested for double heads being somewhat 

 longer than the small grafted piece in the reversed position. If 

 the grafted piece is longer, it produces a tail at its posterior end 

 and not a head. 



Miss Peebles has shown in Tubularia that small pieces grafted 

 on to the ends of large pieces may sometimes take part in produc- 

 ing the single head that develops. This may occur also when 

 the small piece is reversed in direction ; but it cannot be shown 

 in this case that the reversed head is due to its union with the 

 larger piece, because Tubularia produces a new head so readily 

 from either the aboral or oral end that even if some influence 

 of the larger piece exists it would be difficult to prove. 



The pecuHar power of pieces of hydra, planarians, etc., to 

 mold themselves into a new form of typical proportions is clearly 

 similar to the molding that takes place in embryonic develop- 

 ment. The older writers used the term "formative force" 

 to account for this power to undergo changes in form, but 

 modem investigators avoid the use of this term, because no such 

 form of energy is kno^'v^l in the physical world ; and because we 

 should have to postulate as many kinds of formative forces to 



