572 THE WONDER OF LIFE 



octahedron is remotely comparable to the gap between the 

 regenerative hood that forms at the anterior end of a 

 decapitated freshwater worm and the perfectly re-grown 

 head. In both cases there is a gradual series of differentia- 

 tions and integrations connecting the beginning and the 

 end. 



Again, without detracting from the genuine wonder of 

 the facts of regeneration, we may reasonably seek to bring 

 them into hne with analogous phenomena, and we have 

 already referred to asexual multiplication, tissue replace- 

 ment, and the hke. Let us recall also what occurs inside 

 the pupa case of a fly, where the larval body is Uterally 

 disintegrated, and certain minute groups of cells (the 

 imaginal discs) act among the debris as the centres of a 

 reconstruction on an entirely new plan. 



When we think of the earthworm growing a new head, 

 or the hzard a new tail, or the newt a new lens — aU, as it 

 were, at command — we must try to allow for the influence 

 of environing stimuh. The residual germinal power in the 

 animal counts for much, but this operates under the influ- 

 ence of a particular environment, which also counts for 

 much, though not for so much. Perhaps this point may 

 be best understood by reference to what is technically called 

 heteromorphosis, which means that in certain conditions 

 the re-growth departs from its ordinary mode of procedure. 



If an inch or two be cut from the cyhndrical stalk of 

 the common zoophyte Tubularia, and one end be stuck in 

 the sand, a head may be re-grown at the free end whether 

 that was originally turned towards the head or towards 

 the base. But if both ends be left free, the piece re- 

 generates a head at each end. Evidently the environ- 

 mental influences count for something here. There are 



