124 



SENESCENCE AND REJUVENESCENCE 



ten or twelve millimeters the posterior region is more or less clearly 

 marked off by metabolic gradients into two or more zooids 

 (Fig. 29), and the extreme posterior end appears to be a growing 

 tip in which new zooids are arising. In nature, separation at the 



boundary between the first and second zooids 

 very commonly occurs at about this stage, 

 but if the animals are prevented from divid- 

 ing, which may be accomplished in various 

 ways, they may grow to a length of twenty- 

 five to thirty millimeters and the posterior 

 region may consist of four to five zooids and 

 a growing tip (Fig. 30). 



The dynamic demarkation of these pos- 

 terior zooids results, as has been shown else- 

 where,' from a physiological isolation of the 

 regions concerned in forming the dominant 

 head-region of the animal. The consequence 

 of this physiological isolation is the beginning 

 of a new individuation in the isolated region, 

 in essentially the same manner as in the 

 physically isolated piece which begins to 

 undergo reconstitution, and for the same 

 reason. But the physiological isolation of 

 the posterior region of the planarian body is 

 less complete than in the piece isolated by 

 section; consequently the development of 

 new individuation beyond a very early stage, 

 which is only dynamically distinguishable, is 

 inhibited. In Planaria maculata and various 

 other species of Planaria new zooids arise in 

 the same way and exist dynamically as axial 

 gradients, but their morphological develop- 

 ment is similarly inhibited until after their physical separation 

 from more anterior regions. 



The act of fission in these animals results from an independent 

 motor reaction of posterior and anterior zooids. If the animal is 



' Child, '10, 'iia, 'lie; see also chap. ix. 



Fig. 31. — Planaria 

 dorotocephala in process 

 of division. 



