114 HOW ANIMALS DEVELOP 



Once the nerve fibres have got to their destination 

 and become attached, there is a mutual interaction 

 between the nerve and the organ. If the organ is 

 made smaller than normal in any way or is removed 

 altogether, the number of sensory nerve fibres 

 coming back from it carrying sensations up the 

 spinal column to the brain is reduced, although the 

 number of motor nerve fibres going down the spinal 

 column to carry orders to the organ is not affected. 

 On the other hand, the nerve sometimes affects 

 the organ ; thus it is usually impossible for an organ 

 to regenerate if the nerve is cut. In view of this 

 dependence of regeneration on the presence of 

 nerves, it is surprising to find that nerves are quite 

 unnecessary for ordinary embryonic development, 

 and that it is possible to prevent any nerves at all 

 from growing into a leg-rudiment without this 

 having any very great effect in the development of 

 the leg, except that the muscles, although well 

 formed, do not function and therefore do not grow 

 as large as usual. 



Hormones 



Much of the integration of the developing animal 

 seems in fact to be done not by the nervous system 

 but by substances circulating in the blood-stream. It 

 is usual to call these substances hormones, but it is 

 difficult to give an exact definition of this term. They 

 are usually thought of as substances which are 

 present in the blood in very small quantities and 

 which have a specific effect on various organs, but, 



