THEORY OF THE DEVELOPMENT OF ORGANISMS 133 



the egg. The chief factors in the process of 

 development we have found to be : (1) The multi- 

 plication of cells by division (growth as a moulding 

 factor) ; (2) the relations of cells to their external 

 environment (position in its widest sense as a 

 factor); (3) the interrelations of the parts of a 

 whole (cells, tissues, and organs) to one another 

 and to the whole (correlative development). There 

 remains to be considered the extent to which the 

 germinal material in the egg determines the course 

 of development of the organism. Here, before all 

 things, it must be insisted that the individual 

 nature of the cell determines the specific fashion in 

 which the cell will react to the varying stimuli 

 coming from varying conditions. The same agency 

 produces very different results upon different 

 organisms. These differences must be attributed 

 to the differences in the nature (different intimate 

 structure) of the active material. 



Sachs speaks strikingly on this point (Physiology 

 of Plants, p. 602) : ' If the same external cause 

 induces exactly opposite effects in the organs, the 

 explanation of this must simply be sought in the 

 different structure of the organs. If one organ, 

 when illuminated from one side, becomes curved 

 so as to be concave on the side turned towards the 

 centre of light, while another becomes convex on 

 that side, the cause can only lie in the internal 

 structure of the organ. But it is just on such 

 differences of structure that the great variety of 

 reactions which the most different plant organs 

 exhibit towards the same external influences 



