3 22 



CAUSES OF VARIATION 



Lateral regeneration. All examples thus far given are of regen- 

 eration lengthwise, and of the restoration of parts similar to those 

 removed, but arising from matter differently differentiated. 



If the planarian be split lengthwise, either through the middle 

 or well to one side, each piece will regenerate. If a hydra be 



U 



\J 



\J 



FIG. 38. Regeneration in the planarian 



/4, normal worm; B, B l , regeneration of anterior half ; C, C 1 , regeneration of posterior half ; 

 D, crosspiece out of middle ; Z>1, Z> 2 , D%, D 4 , regeneration of same ; , old head ; E l , 

 E2, E*, regeneration of same ; F, old head cut off just behind the eyes ; P* t regeneration 

 of new head on posterior end of same. After Morgan 



split, the edges of the tube will come together and join, making 

 a tube of smaller diameter, enlarging later. If the leg of a sala- 

 mander be split and a part removed, it will be replaced, showing 

 that regeneration is lateral as well as longitudinal. 



If, however, the body of the salamander should be split, both 

 pieces would die, not primarily from lack of regenerating powers 



