SUMMARY AND CONCLUSION 455 



apparatus for division is set in motion by the cell, which is con- 

 trolled by the idioplasm. Were this not the case, the nuclear 

 matter could not be the hereditary substance, for most of the 

 hereditary characters of a species are due in a less degree to the 

 differentiation of individual cells than to the number and group- 

 ing of the cells of which a certain organ or entire part of the 

 body consists ; these, however, again depend on the mode and 

 rate of cell-division. 



The processes occurring in the idioplasm which direct the 

 development of the organism from the ovum — or to speak in 

 more general terms, from one cell, the germ-cell, — do not 

 in themselves furnish an explanation of a series of phenomena 

 which are in part directly connected with the ontogeny, or else 

 result from it sooner or later : the phenomena of regeneration, 

 gem/nation, and fission, and the formation of new germ-cells, 

 all require special supplementary hypotheses. 



The simplest cases of regeneration are due to the fully 

 formed tissue, consisting of similar cells, always containing a 

 reserve of young cells, which are capable of replacing a nor- 

 mal or abnormal loss. This, however, is insufficient in the 

 more complex cases, in which entire parts of the body, such 

 as the tail or the limbs, are regenerated when they have 

 been forcibly removed. We must here assume that the 

 cells of the parts which are capable of regeneration contain 

 ' supplementary determinants ' in addition to those which 

 control them, and that these are the primary constituents 

 of the parts which are to be formed anew in the process 

 of regeneration. They are supplied to certain parts of the 

 body at an earlier ontogenetic stage in the form of 'inactive 

 accessory idioplasm,' and only become active when the opposi- 

 tion to growth has been removed in consequence of the loss 

 of the part in question. The equipment of a cell of any 

 part with supplementary determinants presupposes a greater 

 complexity in their distribution, in correspondence with the 

 greater complexity in structure of the part ; and thus the 

 capacity for regeneration is livnited, for a part can no longer 

 be provided with an apparatus for regeneration when its 

 structure is too complicated. The ordinary assumption that 

 the regenerative ' force ' decreases as the complexity in structure 

 increases, is therefore to a certain extent true, but not if it 

 implies the existence of a special force which provides for 



