THEORIES OF REGENERATION 26$ 



face, but not from the other. He interprets this as due to a polariza- 

 tion of the protoplasm, one surface having peculiarities that are 

 absent in the other. 



There are certain objections to Pfluger's hypothesis that suggest 

 themselves. In the first place the new part does not, in many cases, 

 replace all that has been removed, and hence it is difficult to see 

 how the building up in the way Pfliiger supposes, could take place. 

 In these cases the new material forms only the distal end of the 

 part removed, and the relation of the old to the new part is of sec- 

 ondary importance. Again, in cases of heteromorphosis, as when a 

 tail develops on an anterior cut-surface of a piece of an earthworm, 

 the result must be due to quite different factors from those suggested 

 by Pfliiger. The results are, in fact, the reverse of what the hypoth- 

 esis demands. Furthermore, when the entire piece is transformed 

 into a whole new organism, there is very little in the process to sug- 

 gest a change like that postulated by Pfl tiger. On the other hand 

 there cannot be much doubt that the old part may have some influ- 

 ence, and in certain cases a very important influence on the new part, 

 but whether this is a purely molecular influence is open to doubt. In 

 whatever way this influence may act, it is only one of a number of 

 factors that take a share in the result. The amount of new material, 

 that is formed before the organization of the new part begins, seems 

 to be also a factor; and the one that determines how much of the 

 missing part can be replaced, and this in turn seems to be connected 

 with the lowest organization size that can be produced. The distal 

 end of the new part forms always the distal end of the organ that is 

 to be produced. If enough new material has developed (before the 

 organization of the new part takes place) to produce all of the miss- 

 ing part, the latter is formed, but if the material is insufficient to pro- 

 duce the whole structure, then as much of the distal end as possible 

 is formed. In some cases, as in the planarians, the missing interme- 

 diate regions may subsequently develop behind the distal part that is 

 first produced. 



Sachs has advocated a view which has many points of similarity 

 to that of Bonnet, although, in reality, it is not a theory of pre-forma- 

 tion at all, but one of pure epigenesis. His idea rests on the view 

 that the form of a plant, or of an animal, is the expression of the kind 

 of material of which it is composed. Any change in its material 

 leads to a corresponding change in the form of the new parts. 

 Sachs holds that the idea of many morphologists, that there is for 

 each organism a specific form that tends to express itself, and which 

 controls the development of the organism, is a metaphysical idea that 

 has no ground in science. For instance, Sachs thinks that the flower 

 buds of a plant develop, not because of some innate, mystical force 



