THE AMERICAN NATURALIST. [Vol. XXXVIII. 



In Planaria liigiibris I had found that when the head 'is cut 

 off just behind the eyes a heteromorphic head is produced on the 

 posterior end of the old head. From this result I confess that 

 I anticipated that in this worm it would be even easier than in 

 P. macnlata to obtain double-headed forms from short cross- 

 pieces. To my surprise I found on the contrary that these 

 double-headed forms do not arise except in a few cases in which 

 the cross-pieces have come from the region immediately behind 

 the old head. Most of the other short cross-pieces produce a 

 head and a tail, but in a small number of cases the structure 

 that develops at the anterior end appears to be a tail. It would 

 be difficult to determine with absolute certainty whether this 

 anterior pointed structure is a heteromorphic tail or an undevel- 

 oped head, unless the pharynx were to develop. In one case a 

 ])harynx appeared in the new part at the anterior end, and 

 another in the new part at the posterior end. In both the new 

 pharynx pointed outwards, i. e., towards the tip of the new part, 

 hence there can be little question that the new structure is 

 really a heteromorphic tail. 



It appears in Planaria lugnbris that while the material at 

 every level has the double potency of producing a head or a tail 

 according to which end of the piece it comes to lie at, yet in 

 \ery short pieces from the middle and posterior regions of the 

 body a double-tailed form may arise. We must suppose, there- 

 fore, on our hypothesis, that while in this species also the mate- 

 rial is totipotent, yet when the determining influence of the 

 polarity is removed the stronger tendency is to produce a tail, 

 while in Plaiiario macnlata, as we have seen, the stronger ten- 



In P la nana Ingnbris it has been necessary to assume that the 

 most anterior part of the body has a different predisposition 

 than has the rest of the body, since in the former a double- 

 headed piece may develop and in the latter a double-tailed form. 

 This result finds a parallel in certain other cases that I have 

 observed in the earthworm and tadpole. 



If an earthworm is cut in two behind the region of the gizzard 

 there often develops from the anterior cut surface of the poste- 

 rior piece of the worm a heteromorphic tail. In this case the 



