There are certain objections to Pflüger’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 Pflüger 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 secondary 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 Pflüger. The results are, in fact, the reverse of what the hypothesis demands. Furthermore, when the entire piece is transformed into a whole new organism, there is very little in the process to suggest a change like that postulated by Pflüger. On the other hand there cannot be much doubt that the old part may have some influence, 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 missing part, the latter is formed, but if the material is insufficient to produce the whole structure, then as much of the distal end as possible is formed. In some cases, as in the planarians, the missing intermediate regions may subsequently develop behind the distal part that is first produced.