GROWTH, AGE, AND DEATH 



297 



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tion in regulating development of form (p. 282) then applies to the 

 processes of regeneration as well as in 

 the development of the adult from 

 the embryo. Curiously enough, frag- 

 ments of some planaria, and of other 

 forms as well, may be made to develop 

 two heads, one at each end of the re- 

 generating fragment (Fig. 195). In 

 such cases the polarization, that is, the 

 relation of dominance and subordina- 

 tion which ordinarily determines that 

 one region shall become the head be- 

 cause of its higher metabolic activity, 

 has been obliterated, and both cut 

 surfaces, being equally independent 

 and equally active, regenerate heads. 

 It has been set forth that the ana- 

 tomical characteristics of an animal 

 are determined by the degree of com- 

 plexity of its various organ systems 

 (Chapters V and VI). Similarly, many 

 of the physiological characteristics of 

 an animal are determ.ined by its rela- 

 tive properties of growth, differentia- 

 tion, regeneration, and agamic repro- 

 duction. When differentiation in the 

 adult is sharp, the other three proper- 

 ties are reduced or lacking; differentia- 

 tion in an adult human is high and 

 the animal no longer grows nor re- 

 generates nor reproduces agamically. 

 Thus Man and other mammals in 

 general are regarded as the most highly differentiated forms of 





Fig. 195. — Regeneration of 

 pieces from different regions 

 of a planarian worm. C is a 

 biaxial form; the piece was 

 cut in such fashion that 

 neither end dominated the 

 other during the regeneration 

 of the head; hence two heads 

 were formed. 



