THE CYCLE OF LIFE 391 



is immanent in the nerve-cell, and the initial direction of 

 outgrowth is already determined within the cell before 

 the outgrowth actually begins. The formation of the 

 fibre is therefore an act of self-differentiation within Roux's 

 definition '. 



The second general impression that we get from the 

 study of development is that of a continuous action and 

 reaction between an implicit organization and the environ- 

 ing conditions. We include in the environing conditions 

 not only the external medium and its energies, and the 

 maternal environment where such exists, but also the 

 intra-embryonic environment, the influence of surrounding 

 cells and of the whole on any particular developing unit 

 or area. The developing organism is continually trafficking 

 with its environment, and the result is a function of the 

 intrinsic hereditary nature, on the one hand, and of the 

 appropriate environmental nurture, on the other. 



In thinking of such a difficult problem as embryonic 

 development, it is always profitable to look at it in the 

 light of the development of which we are most immediately 

 aware the development of our own mind and character. 

 Of a truth they are both born and made. Our mind is in 

 great part a social product ; our character has to be wrought 

 out in conduct. What we are aware of is ' the expliciting 

 of the implicit ', the actualizing of something potential, 

 action and reaction between our hereditary nature and a 

 complex environing nurture. Reading back, we feel sure 

 that the same general idea applies to embryonic develop- 

 ment. The general idea is that of the seed which will 

 not germinate except in suitable soil, and duly favoured 

 with sunshine and rain ; but we wish to push the idea back 

 till we see in each cell of the embryo, in each ' organ-forming 



