134 THE ORIGIN OF THE NERVOUS SYSTEM 



as well as other parts and become complete new indi- 

 viduals (Figs. 19-22). In the polyclad Turbellaria, 

 however, pieces from levels posterior to the head are 

 unable to develop new heads, though they regenerate 

 all other parts. In certain other forms, e.g., the triclad, 

 Dendrocoelum (F. R. Lillie, 1901) and various oligo- 

 chaetes (Hyman, 1916), the capacity to regenerate a 

 head exists in the more anterior body regions and dis- 

 appears at a certain level. In one of the rhabdocoeles, 

 Stenostomum, the capacity to regenerate a head exists 

 in the younger asexual stages but disappears when the 

 animals become sexually mature. Moreover, in embry- 

 onic development the head arises earlier than other 

 parts, i.e., when the cells are physiologically younger 

 and their rate of physiological activity higher, and it is 

 localized in the most active region of the organism. 

 And finally, the earlier stages of head development are 

 more susceptible to the action of inhibiting agents than 

 those of other parts of the body. All these facts indicate 

 that a higher rate of metabolic activity in the cells con- 

 cerned is necessary for the development of a head than 

 for that of other parts, i.e., in order to produce a head 

 the cells must be physiologically younger, or must arise 

 from a higher level of the axial gradient and therefore 

 undergo more complete dedifferentiation and attain a 

 higher rate of activity, than is necessary for the develop- 

 ment of other parts. 



Returning to the process of segmentation it appears 

 then not improbable that the cells of the posterior 

 growing region in annelids have already become special- 

 ized to such an extent that they are capable of giving 

 rise only to incomplete headless individuals. In any 



