6 . REGENERATION 



while the other end has a polyp (or head), P (Fig. 12). The 

 writer found that when a piece, ah, is cut out from the stem of a 

 Tubularian and suspended in -sea water, a new polyp, d and c 

 (Fig. 13), is generally formed at either end of the body. In this 

 case the regeneration of the mutilated piece does not result in 

 the restoration of the old organism but in the production of an 

 organism never found under natural conditions, since it termi- 

 nates in a head at either end instead of terminating in a head at the 

 oral and a foot at the aboral end. The substitution of one type 

 of organ by another has been termed heteromorphosis^ and a 

 number of cases of heteromorphosis in regeneration have since 

 been described. 



A still different type of regeneration occurs in plants where 

 shoots originate, as a rule, from preformed definite anlagen. 

 Since this type of regeneration is the subject of this little volume, 

 it need not be discussed in this introduction.^ 



No scientific explanation of these or other cases of regeneration 

 in living organisms has been offered, if by a scientific explanation 

 is meant a rationalistic mathematical theory based on quantita- 

 tive measurements. The explanations offered were either purely 

 verbalistic, such as the assumption of a guiding spirit in each 

 organism which directs its growth according to a preconceived 

 plan (the entelechy of Driesch, the morphaestesia of Noll, etc.), 

 or they were based on assumptions that are plainly in contradic- 

 tion with the facts. One assumption often repeated is that the 

 wound produces specific "wound hormones." It can be stated 

 at the outset that the idea of "wound stimuli" or "wound hor- 

 mones" is excluded in the experiments on Bryophyllum with 

 which this volume is concerned. When a leaf of Bryophyllum. 

 is removed from a plant and submersed with the apex in water 

 (Fig. 14), those notches of the leaf which dip into the water give 

 rise to a new shoot and to new roots. The only wound in this 

 case is at the base of the petiole of the leaf, no regeneration occurring 

 at or near the wound. The growth occurs only in the notches of 

 the apex which in this case is farthest away from the wound. 

 The growth will occur even if a small piece of stem is attached to 



1 LoEB, J. : Untersuchungen zur physiologischen Morphologie der Tiere. 

 I. H eteromorphose, Wiirzburg, 1890. 



2 A collection of observations on regeneration in plants is found in Goebel, 

 K., Einleitung in die experimentelle Morphologie der Pflanzen, Leipzig and 

 Berlin, 1908. 



