104 THE BIOLOGICAL PROBLEM OF TO-DAY 



One may group together as centrifugal coMses 

 of the process of development the characters of the 

 fertilised cells and the interrelations between the 

 products of their divisions, and distinguish them 

 from the centripetal causes, or motive forces that 

 are provided by the action of surrounding things. 

 None the less, it must be borne in mind that there 

 is no sharp distinction between centrifugal and 

 centripetal forces. On page 86 I showed how 

 what is external in one stage of the process 

 becomes internal in the succeeding stage. The 

 external constantly is becoming internal, and the 

 sum of the internal factors increases only at the 

 expense of external factors. 



From the physiological point of view I regard 

 the divergent differentiation of cells as a reaction of 

 the organic material to unlike impelling forces 

 that is, to factors shown by experimental physiology 

 to be actually present and to rule the building up 

 of the organism. ' It were superfluous to detail/ 

 as Naegeli says, * how continually other forces 

 external to the idioplasm, but belonging to the 

 individual, influence the idioplasm; every cell, 

 indeed, as it grows and divides, takes up a definite 

 place in the growing whole, and finds itself in a 

 peculiar combination of conditions of organisation.' 

 ' Not only influences within the individual affect 

 the idioplasm, as that may be altered by external 

 influences, and so may be forced to grow in a new 

 direction. 1 * The influence of surroundings in 

 determining which of the rudiments contained in 



o 



the idioplasm shall achieve development is shown 



