94 INDIVIDUALITY IN ORGANISMS 



the long slope the body of the anterior chief zooid, which 

 forms most of the body of the worm, b represents the 

 apical end of the second zooid, c that of a third, etc. 

 These zooids are the result of successive physiological 

 isolations of the basal region as the animal grows in 

 length. First a single zooid is formed at the basal end, 

 but the range of dominance is short in this undeveloped 

 individual, and as growth proceeds its basal 

 region soon becomes physiologically isolated, 

 and a second zooid arises, and so on. While 

 the degree of physiological isolation is not 



Anterior Posterior 



FIG. 45. Graphic representation of major axial gradients in a 

 Planaria with several zooids: a, head of animal; b, c, apical regions 

 of secondary zooids. 



sufficient to permit the development of the new indi- 

 vidual to proceed very far, some degree of rejuve- 

 nescence in the part does occur and its metabolic rate 

 rises slightly, and with each successive isolation there is 

 a further increase in rate, so that in each successive 

 zooid the gradient is at a level somewhat higher than 

 that of the preceding. 



