THE GENESIS OF LIVING ORGANISMS 217 



With the completion of the framework, function sets in ; 

 function which, on the one hand, seems an outcome of the 

 finished machine, but, on the other, controls it, for its rule 

 is now substituted for the rule of genesis, and, in accordance 

 with plan, guides the course of what happens. 



While in unicellular animals the rule of genesis and the 

 rule of function simultaneously affect the course of the 

 impulses, in multicellulars there is a critical point at which 

 guidance of the impulses passes from the control of the rule 

 of genesis to that of the rule of function. 



THE FINAL STAGES OF GENESIS 



Before we set about determining this critical point, we 

 must consider briefly the further course of development, from 

 the time when the germinal areas are laid down. 



We are indebted to Harrison and Braus for some funda- 

 mental discoveries made from these final stages. It has 

 been shown that the nerves grow out from their germinal 

 area exactly in the same way that amcebse send forth their 

 thread-like pseudopodia. It is important to show that the 

 two processes may be regarded as identical. Moreover, by 

 transplanting nervous germinal areas, it has been demon- 

 strated that the genes furnishing the motor nerves are the 

 same throughout the entire body ; and this considerably 

 restricts the number of different genes. 



If the nerve that arises from the germinal area of the 

 facial has to take the place of the sciatic, it adapts itself to 

 the utterly unfamiliar limb just as well as would the normally 

 developing nerve. If the cells of the engrafted germinal area 

 have the same genes as the normal area, it is all one to the 

 impulse in producing its effect. 



This shows us the complete independence of the impulse- 

 system of an organ from the source of the material on which 



