EEGENEKATIOX 



25 



Walter Voigt lias shown, by a series of most interesting experi- 

 ments, that it is possible not only to cause the development of a new 

 head in Planarians by cutting them, in which case a tail may grow 

 from the anterior portion and a head from the posterior portion, but 

 it is also possible in an intact animal, that is, one with both head and 

 tail, to cause the production of a second head, or a second tail, or both 

 at once, at any part of the body margin at will, according to the 

 direction of the cut. If the margin of the body be cut obliquely 

 forwards (Fig. loo, ^) a supernumerary tail arises (C, s), if it be cut 

 obliquely backwards a supernumerary head arises {C, k), and in this 

 way several heads and several tails \\\a.y be produced in the same 



A 



B 



Fig. ioo. Regeneration of Planarians. A, an animal divided into three 

 parts by two oblique cuts. B, the fragments (a, b, c} in process of regeneration. 

 C, an animal with various oblique incisions in the margin of the bodj^, which 

 have induced the new formation of heads (A), of tails (s), and pharynx {ph]. 

 A and B after Morgan ; C after Walter Voigt. 



animal. It is obvious, then, that the interaction, in the first place, of 

 the cells of the cut surface, but probably also of the deeper-lying cells, 

 decides which determinants are to come into action, those of the head 

 or those of the tail, but both must be present at every part of the cut. 

 How far below the cut surface the cells take part in this deter- 

 mination we cannot make out, but that it cannot be due to the 

 co-operation of all parts is clear in this case at least, since the animal 

 still possesses its original head and tail. The extra heads and tails 

 thus produced prove, at any rate, that there can be no question here 

 of the expression of an adaptive principle, a spiritus rector, or a vital 

 force, which always creates what is good, but that it is rather a 

 purely mechanical process, which takes its course quite independentl}" 



