PROCESS OF REGENERATION 119 



end of the stomach is most susceptible. Chaetogaster differs 

 from other naids in a much lower degree of cephalization, and 

 greater differentiation in the alimentary tract, and these differ- 

 ences probably account for its gradient. 



In the naids, then, the primary gradient is present only in the 

 zooid stage; in the adult individual there is always present a 

 posterior growing region which has a high rate of metabolism. 

 This region varies in length, but usually includes about the pos- 

 terior third of the worm. It does not become independent by 

 virtue of its increased rate of metabolism, but continues to be 

 dominated by the cephalic nervous system as shown by the simple 

 fact that it is stimulated on being isolated from the rest of the 

 worm. This stimulation is demonstrable in two ways, — first, 

 by the increased motor activity after isolation, and secondly, 

 by increased susceptibility of such isolated posterior ends to 

 cyanide. It is obvious that an independent part would not 

 be stimulated by physical isolation from the rest of the organism, 

 for it is already physiologically isolated; thus when the head is 

 cut off, it exhibits no increased activity, and no increased sus- 

 ceptibility to cyanide. 



With the formation of a zooid in the posterior region, its 

 gradient returns to the primary condition. The head of the 

 zooid becomes dominant over this region; its rate of metabolism 

 grows higher and higher as differentiation proceeds; and with 

 the reorganization of the remaining part to form the new ani- 

 mal, a new independent gradient of the primary kind becomes 

 established. 



4. The gradient of Lumbriculus 



In this oligochaete, the posterior rise in metabolic rate is 

 very marked, owing to the greater length which this worm attains 

 (50 mm. or more). A concentration of ^uo to 4^0 KCN is used 

 for this species. Disintegration begins about simultaneously 

 at both ends, or either end may precede, generally the posterior 

 end. The waves of disintegration then sweep rapidly forward 

 from the posterior end and slowly backward from the anterior 

 end, meeting somewhat anterior to the middle of the worm (fig. 



