l8o C. M. CHILD. 



finally cease. Briefly, this means that the primitive excitation- 

 transmission relation in protoplasm must be a gradient with 

 decrement from the region of most intense activity. 



We are accustomed to say that excitation is completely re- 

 versible, and as regards the nerve fiber it may be true, but cer- 

 tainly memory and the possibility of learning demonstrate that at 

 least in certain parts of the nervous system reversibility is not 

 complete. Similarly in muscle the contraction is reversible, but 

 repeated excitation within limits brings about growth, i.e., func- 

 tional hypertrophy. The fully developed organism has reached 

 or approaches a condition of dynamic equilibrium in which most 

 changes except the progressive changes of age are relatively re- 

 versible. Most of the less readily reversible changes have already 

 occurred during development. But embryonic protoplasm stands 

 at the beginning of the developmental process of equilibration, 

 and while the momentary and more superficial excitations are 

 undoubtedly largely or wholly reversible, conditions which per- 

 sist for any considerable length of time must produce modifica- 

 tion in the protoplasmic substratum and this may be more or less 

 permanent, according to the nature of the protoplasm. Accord- 

 ing to this conception, then, the physiological gradients arise as 

 primitive excitation-transmission relations in protoplasm, and if 

 the differential exposure persists long enough they become more 

 or less persistent and constitute physiological axes or radii. 



Objection to this conclusion may be raised on the ground that 

 oxygen supply or rate of removal of CO 2 which have been re- 

 garded as factors concerned in determining gradients are not 

 properly speaking " stimuli " but rather specific material rela- 

 tions between portoplasm and environment. As a matter of fact, 

 however, we know that sufficient differences in oxygen supply or 

 in rate of removal of CO, do determine differences in the rate of 

 the respiratory activities of protoplasm, and the respiratory ac- 

 tivities are apparently essential factors in excitation, particularly 

 in the more primitive excitation processes. As already suggested, 

 many facts indicate that excitation in highly specialized proto- 

 plasms with special structural mechanisms may differ more or 

 less widely from the process as it occurs in protoplasm in gen- 

 eral, but we are concerned here rather with the process in its 



