Experiments in Grafting 295 



In grafting or in splitting individuals, tentacles are sometimes 

 displaced from their normal position, and may come to lie at 

 some distance from the oral end. This result may easily be 

 obtained by cutting off the head just below the line of the ten- 

 tacles. When the new trunk is formed individual tentacles often 

 become drawn out of place (Fig. 19). In all these cases the 

 tentacles become absorbed after a time. The result is impor- 

 tant, because the special conditions in the case of hydra preclude, 

 with much probability, the explanation of the absorption as due 

 to food relations. The tentacles are hollow and open directly 

 into the central digestive space. A tentacle attached at the side 

 is situated in as favorable a place for receiving food as are those 

 of the ring, yet it is absorbed. The result is similar to the 

 absorption of small grafts in the same position, and the expla- 

 nation of the absorption is probably the same in both cases. 



One of the most important results obtained by grafting hydra 

 is the reversal of polarity that can be brought about in the fol- 

 lowing way : If two pieces are united by their anterior or oral 

 ends, and then one of them is cut off near the line of union 

 (Fig. 20), it will often produce, if very short, a mouth and ten- 

 tacles (head) from its exposed aboral or posterior end (Fig. 21). 

 Thus instead of a foot a head develops. This result occurs only 

 when the piece is very short. It seems that the polarity has been 

 reversed, owing to the union with a larger piece. The result is 

 difficult to explain ; possibly the conditions of tension may actu- 

 ally be changed in the smaller piece through the influence of 

 the larger piece, so that the orientation is reversed. The ex- 

 periment needs very careful reexamination before we can safely 

 offer a probable explanation of the result. 



The planarians are also capable of remodeling the proportions 

 of the old parts to produce a new worm of typical proportions. 

 In this case, however, the new head and tail are formed in pro- 

 liferated material, but the rest of the body is remodeled. Ab- 

 normally placed parts, resulting from grafting or from cutting, 

 do not show the power of pinching apart exhibited by hydra, but 

 may after a time be absorbed, especially if small and not sup- 



