AGAMIC REPRODUCTION AND REJUVENESCENCE 123 



that this region has undergone 

 the first step in the process of 

 individuation. Each one of the 

 gradients is the dynamic expres- 

 sion of this individuation. In 

 fact, the body of Planaria, after 

 a certain stage of development, 

 is physiologically a chain of two 

 or more zooids, i.e., of individ- 

 uals organically connected. In 

 young animals four or five milli- 

 meters long only two zooids are 

 distinguishable, the longer, 

 anterior zooid making up the 

 greater part of the body and 

 bearing the head, and the 

 shorter, posterior zooid indi- 

 cated only d3TiamicaUy by a 

 second metabolic gradient in the 

 posterior region. The boundary 

 between the two zooids in these 

 small animals is indicated by 

 the dotted line across the body 

 in Fig. 28. As the animal be- 

 comes longer, other zooids arise 

 in the posterior region by fur- 

 ther physiological division of 

 the original posterior zooid, and 

 when it has reached a length of 



Figs. 28-30. — Development of zooids 

 in Planaria dorolocephala: Fig. 28, a 

 young animal with two zooids, i and 2; 

 Fig. 29, a half-grown animal in which 

 the original posterior zooid has divided 

 into zooids z.i. and 2.2., and 2.2. has 

 vmdergone further division; Fig. 30, a 

 full-grown animal in which still further 

 zooids have appeared. 



2. 



28 



u 



7.7. 



1.2. 



2.1.1. 



2.1. 



2.2. 



2.1.2. 



2.2.+ 



30 



