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TEXT-BOOK OF EMBRYOLOGY. 



degree of complexity is reached, such an arrangement would necessitate an 

 extraordinary number of afferent and efferent neurones or an extraordinary 

 development of branches of each where they connect. Accordingly we find a 

 second category of neurones, the intermediate or central neurones which mediate 



Lumbricus 



Nereii 



Fig. 397. — Illustrating the withdrawal from the surface of the bodies 

 of the afferent peripheral neurones. After Retzius. 



between the afferent and efferent peripheral neurones. These central neurones, 

 together with portions of peripheral neurones in immediate relation with them, 

 form, in all fairly well differentiated nervous systems, including those of all 

 Vertebrates, the central as distinguished from the peripheral nervous system. 



Fig. 398. — A three-neurone reflex arc. van Cehuchten. 

 1, Afferent peripheral neurone; 2, intermediate or central neurone; 3, efferent peripheral neurones. 



The change or stimulus would now pass from receptor through (i) afferent 

 peripheral neurones, (2) intermediate neurones, (3) efferent peripheral neu- 

 rones to effector. This arrangement constitutes a three-neurone reflex arc 



