l6o T. H. MORGAN. 



the anterior end of the worm is cut off a new anterior end is 

 regenerated. This power to produce a new head was found to 

 extend back to about one-third of the length of the worm, i. e., 

 to a region just in front of the pharynx. Behi'nd this level the 

 posterior piece fails to regenerate a head at its anterior end. 



On the other hand, the anterior pieces regenerate a new pos- 

 terior end from any level, with the possible exception of the 

 immediate region of the head itself; but the latter point has not 

 yet been sufficiently examined in this species. It appears a 

 remarkable fact that this planarian should have such extensive 

 powers of regenerating posteriorly, and such limited powers of 

 regenerating anteriorly, especially since, as far as we know, the 

 same cells produce either a head or a tail according to which end 

 is exposed ; but this has not been definitely determined, and 

 would be almost impossible to determine with absolute certainty. 

 Eugen Schultz has also studied the regeneration of Dendroccelum 

 lacteum of Europe * and finds that posterior pieces do sometimes 

 regenerate a head, although the regeneration is very slow, and it 

 may appear that Lillie did not keep his pieces a sufficiently long 

 time for the regeneration to take place. He states, in fact, that 

 most of the posterior pieces died after five or six days. Schultz 

 believes that these posterior pieces have potentially the power to 

 regenerate, but that sometimes the piece closes in such a way 

 that the formation of new tissue is prevented, as I have found to 

 occur occasionally in Bipalium. Lillie, on the other hand, tries 

 to account for the lack of power of posterior pieces to form a 

 head by means of the following hypothesis. He suggests that 

 the regeneration from the posterior cut surface at all levels is due, 

 in some unexplained way, to the presence "of the brain and 

 anterior part of the nervous system in the anterior piece." Con- 

 versely the absence of these structures in posterior pieces is sup- 

 posed to account for the lack of regeneration from the anterior 

 cut surface. A simple experiment would have shown the untena- 

 bility of this point of view. If the head end is cut off just in 

 front of the pharynx so that the brain and the anterior part of the 

 nervous system are removed, and then the tail end of the middle 



1 It has been assumed that the European Dendroccelum lacteum and the American 

 form or forms are identical, but I think this question will bear further examination. 



