158 LIBBIE H. HYMAN 



''fundamental reaction of the species, " to use the phraseology of 

 Child. 



The apical region, or head region, or, in animals which develop a 

 morphologically differentiated nervous system, the cephalic region of 

 the nervous system which is the dominant part of the head is a closer 

 approach than any other part of the organism to a morphological 

 expression of this fundamental reaction system. The fundamental 

 reaction system, dominance of the apical region and the axial gradient 

 are all merely different aspects of the same general idea, viz., that the 

 specific protoplasm of any organism consists fundamentally of a single 

 physico-chemical reaction system. This system is the basis of inherit- 

 ance and its dynamic capacities, the foundation of hereditary char- 

 acters. The first step in organization and in embryonic development 

 results from the establishment in one way or another, of some region or 

 portion of this protoplasmic reaction system as a region of higher rate 

 of dynamic activity. This region dominates development, becomes the 

 apical or head region and determines the axial gradient or gradients 

 which constitute the dynamic basis of polarity and of individuation. 

 The organization and development of various parts of the organism 

 rests upon a similar basis of fundamental reaction system and domi- 

 nance and subordination of parts resulting from differences in rate of 

 reaction (Child, '14 e). 



V. SUMMARY 



1. A gradient in rate of metabolism is demonstrated in the 

 oligochaetes. 



2. In the primary form of the gradient, the rate of metabolism 

 is highest at the head and decreases along the antero-posterior 

 axis. Among the oligochaetes this primarj^ gradient is found 

 only in Aeolosoma, and the zooids of the naids. The primar}- 

 gradient is an integrative gradient. 



3. In the other oligochaetes examined a posterior region of 

 increased metabolic rate exists, and constitutes a secondary 

 gradient superposed upon the pruiiary gradient. The secondary 

 gradient runs in the reversed direction from the primary; it 

 results from the characteristic method of growth of annelids by 

 continuous formation of new segments posteriorly, and is not 

 integrative in character. 



4. In Dero limosa, the secondary gradient involves the pos- 

 terior third of the body; in Lumbriculus inconstans, it includes 

 the posterior half of the body or more; and in the tubifi^ids, it 

 includes all of the body except the first five to fifteen segments. 



