104 THE ORIGIN OF THE NERVOUS SYSTEM 



occurs readily the physiologically isolated part loses to a 

 greater or less extent its differentiation as a part and 

 returns to or approaches an embryonic condition, and if 

 it retains the original axial gradients, or new gradients 

 arise in it in consequence of local conditions, it develops, 

 like any other embryonic protoplasm of the species, into 

 a new individual. In this way the occurrence of 

 budding, fission, and the spatial arrangements of differ- 

 ent axes in multiaxial plants and colonial animals, such 

 as the hydroids, are determined (Child, 19 196). More- 

 over, the evidence indicates that repetition of parts in 

 series, e.g., segmentation (Child, igijd; chap, viii, 

 below), and at least many other repetitive develop- 

 mental phenomena are essentially similar in character. 

 In short, any active region, whether it be the apical end 

 of a whole organism or a special organ, such as a hydroid 

 tentacle, dominates to some extent a certain region 

 about it so that only outside of this region and beyond 

 the range of this dominance is a repetition of the indi- 

 vidual part or organ possible. 



According to this conception, then, the gradients 

 established in protoplasm may determine not only the 

 course of differentiation along an axis but in the simpler 

 organisms the limit of size of the individual, the occur-, 

 rence and order of various agamic reproductive processes, 

 and the orderly repetitive arrangement of parts. There 

 is much evidence to indicate that the range of dominance 

 increases greatly during individual development, par- 

 ticularly in animals in which a nervous system with a 

 much higher conductivity than protoplasm in general 

 develops. Physiological isolation of parts may occur in 

 embryonic stages when the organism is very small, but 



