"Polarity:' 503 



It must be assumed, of course, that while some materials grade 

 off from head to tail others grade off from tail to head, etc. Thus 

 in Lumbriculus, the head-end material grades from the anterior 

 end backward, while the tail-forming materials decrease from 

 behind forward. There must often be regions of considerable 

 overlapping of these materials as shown again in Lumbriculus 

 and Tubularia and less so in Lumbricus. Furthermore certain 

 regions may consist so largely, or even exclusively, of certain kinds 

 of substances that despite the postulated polar gradation these 

 regions are capable of regenerating only one kind of structure. 

 Thus as I have shown in the posterior regions of the earthworm, 

 and in the tail of the tadpole, only one kind of structure, e. g., tail, 

 can develop. I have tried also to show that the heteromorphosis 

 of very short cross-pieces of Planarians finds its best explanation on 

 the assumption that by the partial removal of the polar influences 

 in such short pieces, this influence no longer dominates the develop- 

 ment, and the centripetal influences determine that a new head 

 will develop — the head-forming substances being in excess. In 

 other Planarians the tail-forming substances seem to be dominant 

 in certain parts and a heteromorphic tail develops at the anterior 

 end when the polar influence is lessened. 



The most striking example of the influence of the gradation 

 of the material on the formative action is shown by lateral pieces 

 of Planarians. If cross-pieces are first cut from the anterior, 

 middle, and posterior regions of a Planarian, and if the sides are 

 then cut from these pieces, so that none of the median organs are 

 left, it will be found that the position of the pharynx in the new 

 worms that regenerate will depend on the region of the original 

 worm from which the pieces were cut. In the new worms from 

 lateral pieces from the anterior part of the original worm the 

 pharynx lies near the posterior end, in the worms from the middle 

 pieces the new pharynx lies in the middle of the length; and in 

 the worms from posterior pieces the new pharynx lies nearer the 

 anterior end of the new worm. Thus, although in all these pieces 

 having the same shape (and open at both ends) the possibility 

 would seem to exist for the pharynx to lie in the same place in 

 the new worm, in reality its position is different, and appears to 

 be determined by the gradation of the material. Thus in the 

 anterior pieces there is more of the pharynx material in the part 

 of the piece nearer to the old pharynx, /. e., at the posterior end. 



