2 26 THE GERM-PLASM 



the cell in which they are situated, and not as regards develop- 

 ment as a whole. 



Various circumstances may, however, produce complications 

 in this simple course of development of the idioplasm. 



In the first place, it is necessary that the organism should be 

 able to replace losses of substance. In order that this may be 

 possible, the final cells of ontogeny, at any rate, — i.e.^ those of 

 the various tissues, — must be rendered capable of producing 

 others similar to themselves. The possession of this power 

 necessitates that each cell shall be capable of unlimited multi- 

 plication, so that the single determinant which controls it can 

 likewise grow and multiply. This would result in the cells being 

 able to produce others of a like character by cell-division. 



We find, however, that regeneration is not limited to this very 

 simple form of restoration : more complex tissues can also be 

 reproduced ; and even entire organs, such as limbs, and still 

 larger parts of the body, such as the head and tail, may, in 

 certain groups of animals, undergo restoration when they have 

 been lost. These facts are to be explained in terms of the idio- 

 plasm by supposing that in these cases also the determinants for 

 the groups of cells which are to be capable of regeneration have 

 undergone an increase, and have been supplied to certain cells 

 in the course of ontogeny in the form of inactive accessory idio- 

 plasm. The equipment of such cells with determinants for 

 regeneration is due to adaptation, and is only connected with 

 the degree of organisation of the animal in so far as the difficulty 

 of providing a large number of cells with accurately graduated 

 determinants for regeneration increased as the number of cells 

 from which regeneration had to proceed became larger, and as 

 the number and degree of differentiation of the organs to be 

 restored also increased. An increase, both as regards cells and 

 organs, takes place in correspondence with the complexity of 

 structure, and the ' regenerative power/ therefore, as a rule, grad- 

 ually undergoes a proportionate decrease. 



Cells which possess the regenerative power are therefore those 

 which contain, in addition to their own active determinants, a 

 larger or smaller group of inactive determinants : these latter 

 belong to those cells and cell-series which are capable of taking 

 part in the reconstruction of that part of the body which has 

 been lost, and which is situated distally to them when the 

 determinants become active. The occurrence of regeneration in 



