74 CM. CHILD 



lieving that conduction occurs more readily and to greater dis- 

 tances in the downward direction of the metaboHc gradient. 

 Conduction against the gradient is possible but requires a much 

 stronger stimulus for a given distance. If the nerve impulse 

 is a wave of chemical reaction, as seems now to be demonstrated 

 (Tashiro '13), it is at least probable that in order to travel for any- 

 appreciable distance up the original gradient, the impulse must 

 be strong enough to reverse the gradient temporarily as far as 

 it proceeds, while a much weaker impulse may travel down the 

 gradient for long distances. But whether this interpretation 

 is correct or not, the fact remains that in planarians and many 

 other lower forms, nervous and dynamic correlation (Child 

 '11 a, p. 18) in general, is chiefly from more anterior to more 

 posterior regions. 



Admitting this fact, it follows that cells at the anterior end of 

 a piece are in general more independent of other regions of the 

 piece than the cells of any other level. It has been pointed out 

 that the conditions necessary for head-formation are: first, 

 embryonic cells of the species, second, physiological or physical 

 isolation from correlative factors, and third, a sufficiently high 

 rate of metabolism. It is evident that the condition of isolation 

 from correlative factors is more completely fulfilled at the ante- 

 rior end of the piece than elsewhere. At the posterior end, on the 

 other hand, the new cells can never become physiologically 

 isolated unless the axial gradient is eliminated, and in such cases 

 we find that a head may actually arise at the posterior end of a 

 piece. And in cases where the gradient is more or less perma- 

 nently reversed, a tail may arise at the anterior end. 



Briefly stated, the localization of new head and tail on a piece 

 are due to the existence of the axial gradient. The head, which 

 develops independently of other parts, develops at the anterior 

 end because this is the only region of the piece where a sufficient 

 degree of physiological isolation can possibly occur, as long as 

 the axial gradient persists. The tail, which arises only as a 

 subordinate part dependent upon and determined by more 

 anterior regions, is localized at the posterior end of the piece 

 because the cells in this region cannot become physiologically 



