106 C. M. CHILD 



body in the uninjured worm, but the differences in capacity for 

 and rate of head-formation are related to this gradient only 

 indirectly. Moreover, the rate of the dynamic processes in iso- 

 lated pieces is not the same as before isolation and the effect of 

 isolation on the rate of reaction differs in a characteristic manner 

 for different levels of the body. The axial gradient which appears 

 in the regulation of pieces taken in order along the axis is of course 

 related to the dynamic axial gradient existing in uninjured worms, 

 but as will appear more clearly later, the relation is not direct and 

 immediate. 



To sum up ; the comparison of regulation of pieces from differ- 

 ent levels along the axis and under constant external conditions 

 indicates the existence of some sort of gradient or gradients in 

 the uninjured animal, but gives us no direct evidence concerning 

 the nature of the gradient. 



B. THE AXIAL FACTOR IN THE REGULATION OF PIECES UNDER 

 EXPERIMENTAL CONDITIONS 



If an axial gradient does exist in the planarian body, such a 

 gradient must either persist as a fraction of the gradient in the 

 parent body or arise de novo in every piece that regulates into a 

 new whole and the possibility exists of obtaining further evidence 

 concerning the nature of the gradient with the aid of experimental 

 conditions. For example, if the rate of reaction in morphogen- 

 esis at the posterior end of a piece of Planaria is appreciably 

 less than that at the anterior end it should be possible with the 

 aid of external factors which decrease the rate of reaction, to 

 reduce the rate at the posterior end to such a level that morpho- 

 genesis would cease or almost cease there while it still went on 

 at an appreciable rate in more anterior regions. 



As a matter of fact it is possible to demonstrate the • axial 

 gradient in this manner with various external factors, e.g., the 

 anesthetics alcohol, ether chloretone, etc., low temperature, meta- 

 bolic products in the water, potassium cyanide, lack of nutrition 

 and other factors which decrease the rate of the reactions in 

 the organism. The effects of all the conditions mentioned are 



