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EVOLUTION OF ANIMALS 



Part V 



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Fig. 25.7. Regenerating planarians. A, their capacity to regenerate is greatest 

 at the anterior end; B, a regenerating piece shows its natural polarity, that is, 

 the head grows from the front and the tail from the rear as it does in normal 

 animals; C, a piece removed from the head and grafted into the body produces a 

 head; D, a short piece taken near the head may regenerate a head at each end. 

 (After Child: Patterns and Problems of Development. Chicago, University of 

 Chicago Press, 1941.) 



The results of experiments upon planarians support C. M. Child's theory 

 of the axial gradient. This theory postulates that there are different rates of 

 metabolic activity in different regions of an animal's body, commonly the 

 highest at the anterior and lowest at the posterior end. Planarians confirm this 

 since pieces taken from the front end of a planarian grow faster and larger 

 than those taken from the rear. In some species, only the pieces from the front 

 will produce heads. Experiments show that the head dominates adjoining 

 regions and leads them to cooperate in their growth. If the central part of the 

 head of one planarian is grafted into an open wound in another planarian, it 

 will not only develop a whole head, but will influence adjacent tissues to pro- 



