IKDI\TDUATIOX AXD REPRODUCTION 225 



ducting paths of the nervous system, while in other parts of the 

 body the metaboKc rate has been altered by various factors. 



At present there seems to be no good reason for beheving that 

 the changes or impulses transmitted from the dominant region 

 affect the metabohc processes in regions which they reach ia any 

 other than a quantitative way. The dominant region is not to be 

 conceived as giving rise to a variety of different kinds of impulses 

 which produce different, specific, formative effects, but rather 

 merely as a region of high metabolic rate, from which changes con- 

 nected with its metabohc activity spread or are transmitted to 

 other regions and iacrease their metabohc activity. Since these 

 transmitted changes decrease in energy or effectiveness with trans- 

 mission, they must determine a higher rate in the regions nearer 

 the dominant region than in those farther away. In this way the 

 determination of a high rate of metabohsm in one region may result 

 in the estabhshment of a metabohc gradient in one or more direc- 

 tions from that region. Each point along an axis is then character- 

 ized by a more or less definite rate of metabohsm, and if more than 

 one axis is present each point in the organism has a rate determined 

 by its position in each of the axial gradients. 



From this point of view the axiate individual, whether it is a 

 whole organism or a part, when reduced to its simplest terms con- 

 sists of one or more gradients in rate of metabohsm in a cell or cell 

 mass of specific constitution. Of course this condition represents 

 only the first step in individuation. Whether every individual 

 organism in every generation has its beginning in a condition as 

 simple as this can be determined only by extensive investigation. 

 Certainly other factors, such as difference of conditions at the sur- 

 face and in the interior, the presence of reserve substance such as 

 yolk in certain cells, etc., play a part sooner or later in many cases. 

 But that the simplest axiate individuals among organisms consist 

 essentially of metabohc gradients in a specific protoplasm is a 

 conclusion supported by a large body of evidence. The axes of 

 the organism or its parts are, according to this view, in their simplest 

 terms nothing but such gradients, and the structure of the apical 

 region or head of the organism represents merely the develop- 

 mental result of a high rate of metabolism and independence of 



