REGENERATION 35 



conditions, to a whole new plant; every tree and every higher plant 

 has a multitude of cells in its leaves, its flowers, and so on, which 

 cannot do this, which are in fact differentiated in one particular 

 direction, that is, they contain only one kind of determinants, like the 

 histologically differentiated cells of the tissues of the human body. 

 Secondly, there are other organisms besides plants, and a theory of 

 development cannot be based on the phenomena to be observed 

 among plants alone, any more than a theory of heredity can. There 

 are obvious differences in the processes of life among plants as 

 contrasted with those among animals, but it is improbable that 

 there is any thoroughly fundamental diiference. It is, however, 

 indubitable that the cells forming the tissues of higher animals, the 

 nerve, muscle, and glandular cells, are really differentiated in one 

 direction, and are quite incapable, under any circumstances whatever, 

 of growing into an entire organism, and even from this alone we might 

 conclude that they contain only one primordium or determinant. Are 

 we then to assume that the vascular cells, epidermis-cells, wood-cells, 

 and so on, of the higher plants, which are also differentiated in one 

 direction, do nevertheless contain the complete germ-plasm ? I do not 

 see any ground for such an assumption. 



To conclude what can be said on the subject of regeneration we 

 must return to the question of an ultimate explanation of this marvel- 

 lous phenomenon. I have declined to attempt any explanation at all, 

 because I do not consider it possible to give a sufficient one as yet, 

 but I should like at least to give an indication as to the direction 

 in which we must look for it. 



We assumed that there is a regeneration-idioplasm, and therefore 

 that there are ' primary constituents ' at certain positions in the 

 body, but how does it happen that these are able to build up the lost 

 parts in the proper situation and detail ? A theoretical formula 

 might well be thought out, according to which the determinants of 

 successive parts would become active successively, and would thus 

 liberate one another in an appropriate order of sequence, but there 

 would not be much gained by this, especially as what we already 

 know in regard to the regrowth of the legs and toes in Triton does 

 not harmonize with such an assumption. It appears to me more 

 important — though even here we must still be very vague as to details 

 — to recognize that, in ail vital units, there are forces at work 

 which we do not yet know clearly, which bind the parts of each unit 

 to one another in a particular order and relation. We were obliged 

 to assume such forces even in regard to the lowest units, the biophors, 

 since otherwise they could not be capable of multiplication by 



D 2 



