PROCESS OF EEGENERATION 



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extent that the axial gradient of the anterior zooid is reversed 

 (fig. 7) ; here disintegration begins at the posterior end and pro- 

 ceeds to the anterior end. This condition is found in individuals 

 in which the posterior end has grown considerably without be- 

 coming physiologically isolated as a zooid; in such cases, the 

 increasing youth of the segments posteriorly, as well as the fall 

 in rate of metabolism of the anterior end through senescence con- 

 tribute to cause the reversal of the gradient. As soon as the 

 posterior end of such an individual becomes isolated as a zooid, 

 the primary gradient reappears in it. 



Fig. 7 Graph of an Aeolosoma with a zooid, showing reversal of the gradient 

 in the principal zooid. 



As a further interesting detail, I may add that although the 

 disintegration of the head usually begins at the tip of the prosto- 

 mium, yet sometimes, and especially in old individuals, the first 

 region of the head to disintegrate is the ciliated pharynx. Such 

 a specialized organ by virtue of its sensory functions, ciliary 

 activity, etc, must possess a relatively high rate of metabolism 

 which it retains when the rate of surrounding parts has dimin- 

 ished through senescence. Such specialized parts with high 

 susceptibility to cyanide are frequently met with in disintegration 



