Nos. 451-452.] AMERICAN SOCIETY OF ZOOLOGISTS. 503 



head on the anterior end of the posterior piece. It appears 

 that the same material is doubly potent, being able at nearly 

 every level to form a head or a tail. Something in the piece 

 itself determines that a head shall develop at the anterior cut 

 surface and a tail at a posterior cut surface. This "something" 

 is what we call polarity. 



I have found certain exceptions in the planari;uis to this rule. 

 Occasionally a cross-piece develops a head at each end, and in 

 Planaria lugubris, especially, when the tip of the old head is eut 

 off just behind the eyes, another reversed head regenerates troni 

 its posterior cut surface. 



In the few cases in which I got double-headed forms I was 

 impressed by the fact that this occurred only when the cross- 

 pieces were very short, and never occurred in long cross-pieces. 

 This lead me to try making the pieces very short in cnder to sec 

 if I could obtain in this way more of these double-headed tornis. 

 This I found to be the case, and I have obtained from such 

 pieces double-headed forms from every part of the body of 

 Plmiaria maailata. It is clear, therefore, that the result can 

 not be connected with the presence of any particular structure 

 at the cut end. A number of different kinds of experiments, 

 mainly with pieces having obliquely cut ends, seemed to show 

 that the main factor in the production of the heteromorphic 

 head is connected with the shortness of the piece. I suggest as 

 a tentative hypothesis that this shortening of the piece reduces 

 the strength of the polarity so that it comes to ha\e less inHu- 

 ence on the new part than certain innate tendencies of the mate- 

 rial itself. There is reason to conclude, as I have stated, that 

 the new material that develops over the cut surface has the 

 power of producing either a head or a tail. If now the po aiit\ 

 is reduced, or removed, that one of these two tendent les whu 1 

 is stronger will have a chance to assert itselt. In the case ot 

 Planaria macnlata the tendency to pr(xluce a head seems 

 stronger than the tendency to produce a tail, hence the W^^r- 

 ance of the heteromorphic head " ' "'^ " 

 which the polar influences fail to 

 subject to future 



the nature of the so-called polarity itself. 



hold 



vhen we have gamed 



