42 MARINE BIOLOGICAL LABORATORY. 



an eligible witness to the metamerism of the verte- 

 brate head. 



We are indebted mainly to recent studies on the 

 development of the nervous system for the views now 

 held on this subject. In the trunk we find each seg- 

 ment provided with a pair of so-called spinal nerves, 

 both of which spring from the spinal cord by two 

 short roots, known as the anterior and the posterior 

 root. The posterior root bears, just before its union 

 with the anterior root, a spinal ganglion, and is thus 

 stamped as something different from its fellow. This 

 anatomical distinction is the basis of a physiological 

 distinction, the discovery of which, by Sir Charles Bell, 

 in the early part of this century, has been regarded as 

 the most important acquisition of physiology since the 

 time of Harvey. Bell determined by experiment that 

 the posterior roots are appointed for sensory, the an- 

 terior roots for motor, work. Thus both structure and 

 function suggest that the spinal nerve is not one nerve, 

 but two nerves united ; and this point is settled beyond 

 dispute, first, by the independent and unlike develop- 

 ment of the two roots, and second, by their complete 

 and permanent separation in such fishes as Amphioxus 

 and Petromyzon. Each segment of the trunk may 

 therefore be said to have two pairs of nerves, a sensory 

 pair with ganglia, and a motor pair without ganglia. 



Now we come to a question of absorbing interest not 

 only to the embryologist but also to the anatomist and 

 physiologist. Are the metameric arrangement, the divi- 

 sion of labor or function, and the mode of development, 

 essentially the same for the cranial as the spinal nerves? 

 The several inquiries into which the question resolves 



