290 Experimental Zoology 



not produce the typical form, so that a new whole of the charac- 

 teristic type results. The difference in these two classes of cases 

 is due to the different kinds of regenerative changes of which 

 the animals are capable. Pieces of hydra and of planarians 

 can transform themselves into the typical form with very little 

 production of. new tissue, and it is owing to this power that the 

 changes about to be described are due. In the other animals, 

 the tissues, when once formed, cannot remodel themselves into 

 a new whole. At most they can only replace lost parts by first 

 proliferating new material at the cut surface. The experiments 

 to be described are the results of the work of Trembley, Wetzel, 

 King, Rand, Peebles, and others. 



When the anterior half of a hydra is grafted to the posterior 

 half of a hydra, a single individual of normal proportions results, 

 and no further changes take place (Fig. 21) ; but the result is dif- 

 ferent if the united pieces are shorter or longer than a normal 

 hydra. If the anterior piece is less than half and the posterior 

 piece is less than half, a short hydra is produced (Fig. 2). In 

 this case the combination grows longer from day to day until 

 the normal proportions are reached and a hydra of typical form 

 results. If, on the other hand, both the anterior and the pos- 

 terior pieces are each more than a half, a "long" hydra results 

 (Fig. 3). The only way in which such an animal could become 

 normal in length would be by the absorption of some of its 

 parts, but it appears that hydra does not follow this method of 

 remodeling. Instead it regenerates at the line of union a new 

 foot that belongs to the anterior part, and new tentacles at the 

 line of union that belong to the posterior part (Fig. 4). The two 

 parts pinch in two between the new foot and the new tentacles, 

 and two hydras result. Evidently the conditions in a "short" 

 hydra and in a "long" hydra are different, since different results 

 follow. In both cases we must suppose that the union between 

 the cut ends is perfect, so that the result depends on some other 

 relation in this region than imperfect union. The most plausible 

 explanation is found, I think, in the adjustments of the pressure 

 or tension relations of the united pieces. In a "short" hydra 



