ATROPHY 125 



each half-head completes itself (as Trembley had already shown). The 

 body then begins slowly to separate into two parts, beginning at the 

 angle between the two heads, until finally the two parts completely sep- 

 arate. King (1900) has repeated the experiment in a large number of 

 cases with the same result. It seemed that the division might be 

 brought about by the weight of the halves causing the gradual sepa- 

 ration of the body, but King has shown that this is not the case, for, 

 when a double form remained hanging with its head down, it still 

 divided into two parts (Fig. 47, A). In this case, the weight of the two 

 heads would cause the parts to come together rather than to separate, 

 if gravity had any influence of the sort suggested. Marshall and 

 King have also shown that if the posterior end of a hydra is split in 

 two, the two parts do not continue to separate, but one of the two, if 

 the pieces have been split some distance forward, may become con- 

 stricted from the other, and, producing new tentacles at its apical 

 end, become a new individual. 



I have carried out a series of experiments on planarians of a some- 

 what similar nature. If the posterior end is split in two, the separa- 

 tion extending into the anterior part of the worm (Fig. 44, C\ 

 each half completes itself, but the halves do not separate unless they 

 happen to tear themselves apart. If one of the pieces is cut off, not 

 too near the region of union with the other half, a new posterior end, 

 replacing that cut off, regenerates. If, however, the piece is cut off 

 quite near the region or union of the halves, the piece that is left 

 may be absorbed. 



The absorption of misplaced parts in the lower animals cannot be 

 explained, I think, by any lack of nutrition, especially in the case of 

 the tentacles of hydra. The result may be due either to the displaced 

 part not receiving exactly those substances, perhaps food substances, 

 that it gets in its normal position, or it may be due to some formative 

 influence. At present we are not in a position to decide between 

 these alternatives, and, while the former view seems more tangible, 

 and the latter quite obscure, the latter may nevertheless be found 

 to contain the true explanation. If the view that I have adopted in 

 regard to the organization namely, that it can be thought of as 

 acting through a system of tensions peculiar to each kind of proto- 

 plasm is correct, it may be possible to account for the absorption of 

 misplaced parts by some such principle as this. 



INCOMPLETE REGENERATION 



A somewhat unusual process of regeneration takes place when 

 the jelly-fish, Gonioncmtts vcrfcns, is cut into pieces. As first shown 

 by Hargitt, the cut-edges come together and fuse, and the pieces 



