2O2 BIOLOGICAL LECTURES. 



dence of the fact that the chemical composition of the various 

 organs of the body is different, and I think it is not going too 

 far to claim with Sachs that in a sense their form is the out- 

 come of their composition. We have also many observations 

 to show that during regeneration like parts form like. In the 

 earthworm, for instance, the new ectoderm comes from the old, 

 the new digestive tract in large part from the old one ; the new 

 nerve cord comes, in part at least, from the old one and in part 

 from the ectoderm. In other words, the specific character of 

 the old cells may be handed over to the new ones. If we 

 choose to think of a formative substance in each kind of cell, 

 we could make this the basis of our interpretation of the results ; 

 but since we do not know of such formative substances, it is 

 safer to rest our claim simply on differences in the chemical 

 substance of the cell itself. In other words, we only complicate 

 our view by assuming formative substances acting on an indif- 

 ferent medium the protoplasm and determining its changes. 

 We need not deny that this might sometimes happen, and, in 

 fact, several cases that Sachs has cited seem to be due to some 

 such action ; but, in general, it is not necessary to make this 

 distinction, at least not in most cases where regeneration takes 

 place. 1 



The assumption, in those cases in which regeneration takes 



1 By composition of a cell or of any part of the body, we may mean either of 

 two things. The fundamental substance may be thought of as everywhere the 

 same, *'.<?., indifferent, and its action is directed by grosser substances (formative 

 substances), or we may mean by composition the entire substance without making 

 any distinction between the substances of which it is made up. For the latter alterna- 

 tive we may use the terms " chemical " or " molecular " structure or " constitution," 

 and retain the term " formative substance " for the former alternative. For example, 

 we may think of a growing point as made up of indifferent protoplasm, and its fate 

 determined by the kinds of formative substance carried to the growing point (Sachs). 

 When new substance is added during regeneration at a cut surface, we may think 

 of the new cells as at first indifferent, and their fate determined by the formative 

 substances transported to or acting at the cut surface, or we may think of the new 

 cells as having a molecular structure like that of the cells (or parts) from which 

 they have come. This molecular structure might be thought of as changing later, 

 at least so far that at different levels in the new part it changes in its relation to 

 what is around it, and forms a new whole. The change would be due to some 

 other factor or factors, such, for instance, as the form of the new part or its 

 relation to the old part. 



