15 A nerve cell of the mammalian spinal chord and the associated 

 nerve fibre. This is not a photograph but a diagrammatic representa- 

 tion which shows the principal features. The little black 'worms' are 

 mitochondria and the grey ones are microsomes. The open circles 

 are fat-bearing particles. (From an article by J. Z. Young, Endeavour, 

 15, 5, 1956, reproduced by permission.) 



