68 LIBBIE H. HYMAN 



are more susceptible than those of lower rate. It is futile and 

 unnecessary to inquire whether this relation is direct or indirect, 

 owing to our ignorance as to the way in which toxic agents act 

 on protoplasm. The evidence is in my opinion quite sufficient 

 to warrant the conclusion that a differential susceptibility indi- 

 cates a differential metabolic fate. The results may then be 

 translated into metabolic terms as follows: a local increase in 

 metabolic rate occurs, resulting in the beginning of a pseudo- 

 podium, and the protoplasm continues to flow out as long as the 

 metabolic increase persists with its original intensity at the ad- 

 vancing tip. A metabolic gradient thus results along the axis of 

 each pseudopodium. As soon as the metabolic rate at the 

 distal end of the pseudopodium falls below a certain value, move- 

 ment ceases, as shown by the fact that non-growing pseudopodia 

 are less susceptible to cyanide than moving ones. In contem- 

 plating these phenomena one is irresistably reminded of the 

 metabolic 'blaze' which accompanies (precedes?) muscle con- 

 traction, nerve conduction, gland secretion and protoplasmic 

 activities in general. 



One may say then that metabolic gradients exist in the amoeba 

 as in other organisms but that these gradients are temporary 

 and readily appear and disappear. The advancing end or ends of 

 the amoeba have the highest metabolic rate of any part of the 

 organism and hence correspond to the anterior ends of other 

 organisms. Clearly in the amoeba an anterior end is differen- 

 tiated physiologically but not structurally; and this physiological 

 differentiation if it should become permanently established would 

 obviously be a basis for a morphologically differentiated head. 

 Thus the metabolic gradient is the cause of the antero-posterior 

 axis, and even in the amoeba its presence at once brings about 

 a certain structural gradation in the physical condition of the 

 protoplasm, as will appear later in this paper. 



I am aware that various criticisms can, be made against this 

 interpretation of the experimental results. I am unable, however, 

 to conceive of any explanation of the observed facts which must 

 not postulate an internal difference of some sort along the axis 

 of the pseudopodium. Be this postulated difference one of 



