REGENERATION IO7 



sors of this cell which contained muscle-determinants should 

 take up their position in the precise region of the limb in which 

 this particular muscle is situated. 



We must not, however, imply from what has been said above, 

 that external influences are of no importance whatever in ontog- 

 eny, but merely that they certainly only play a secondary part 

 in the process. A limb will certainly grow crooked if a corre- 

 sponding external pressure is brought to bear upon it. Growing 

 cells do not cease to multiply directly they are subjected to 

 abnormal external influences, for they can accommodate them- 

 selves to circumstances. It is such cases as the regeneration of 

 broken bones and the formation of new joints under abnormal 

 external conditions, which prove that the cells continue to 

 perform their functions of growing and of giving rise to organs 

 under circumstances which deviate very markedly from the 

 normal. These false joints also show what a considerable 

 power of adaptation is possessed by the cells, and how efficient 

 may be the parts which these cells are able to produce under 

 abnormal conditions. But although the principle formulated 

 by Roux * of the struggle of the parts, or as it might well be 

 called ' intrabiontic selection,^ is certainly a very important one, 

 I think it would be a great mistake to refer the normal process of 

 ontogeny mainly to this principle. The groups and masses 

 of cells must certainly press upon one another durin<; the process 

 of differentiation : in the process of the formation of a joint, for 

 instance, proliferating connective-tissue cells do actually force 

 themselves amongst the cartilage cells in one part of the rudi- 

 mentary bone, in order to separate them from one another. But 

 this proliferation and pressure are taken account of, just as much 

 as are the processes of dissolution or absorption that occur in 

 those cells in the primordial cartilage which are situated in the 

 region of the joint. It might be supposed that the existence 

 of so-called 'identical' human twins contradict mv conception 

 of ontogeny ; for although they are undoubtedly derived from 

 a single ovum and sperm-cell, and hence possess the same kind 

 of germ-plasm, they are never really identical, but only very 

 similar to one another. But apart from the fact that the abso- 

 lute identity of the germ-plasm has not been proved in these 

 cases, the very close resemblance between these twins shows 



* W. Roux, ' Der Kampf cler Theile im Organismus,' Leipzig, 1881. 



