NERVOUS SYSTEM AND OTHER PARTS 123 



of these facts has been discussed elsewhere and it has 

 been shown experimentally that the development of the 

 regions normally intermediate between the head and 

 the level represented by the piece is determined by the 

 presence of the developing head (Child, 191 ib, 1915c, 

 pp. 102-17). Similarly, the development of a new 

 posterior end at the posterior end of the isolated piece 

 is determined by the regions anterior to it and cannot 

 occur independently of them, though it may occur in 

 the absence of a head (Figs. 8-1 2) . The development of 

 the new head, however, actually occurs in spite of the 

 rest of the piece and may under certain conditions be 

 more or less completely inhibited by the rest of the 

 piece (Child, 1914^, e, 19166, 1920). 



The most striking evidence for the physiological 

 independence of the apical end or head is found in the 

 regulatory development of very short pieces. Such 

 pieces give rise to apical or cephalic structures alone, 

 all other parts of the body being entirely absent. Short 

 pieces of Tubular ia stem (Figs. 23-29) from any level 

 may give rise to single or biaxial whole hydranths or 

 partial hydranths depending on its length, and short 

 pieces of the planarian body may give rise to single or 

 biaxial heads (Figs. 30-34). The frequency of biaxial 

 structures in these short pieces is due to the fact that 

 the short piece, representing a very small fraction of 

 the polar axial gradient, is almost apolar, and in many 

 cases the exposure and the presence of the wound at the 

 two ends is sufficient to determine two opposed gradients 

 and so a biaxial development. These apical and cephalic 

 structures which develop in the complete absence of 

 other parts may be complete and normal in all respects 



