138 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The close association of sensory and motor nerves in the trunk region of verte- 

 brates is not yet satisfactorily explained. The fact that in Amphioxus the two 

 kinds of nerves are independent of each other throughout their course shows that 

 the vertebrate condition is not primitive. 



The infolding of the nervous plate has been described (p. n) 

 and with that stage the present account begins. As the plate is broad- 

 est in front, the result is a larger anterior portion of the tube, the brain, 

 while the rest of the tube gives rise to the spinal cord. Brain and 

 cord constitute the central nervous system, while the nerves arising 

 from the brain and cord form the peripheral nervous system. 



CENTRAL NERVOUS SYSTEM. 



The two halves of the neural plate are separated by a median band 

 of non-nervous tissue, hence, when it is rolled into a tube, the mid- 

 ventral line the floor plate is thinner and differs from the side walls. 

 With the closure of the tube (fig. 144, A) a similar roof plate appears, 



FIG. 144. A, diagram of early spinal cord; B, later, showing increase in size and con- 

 sequent ventral fissure, c, central canal; e, ectoderm;/, floor plate; g, anlage of spinal 

 ganglion; nc, neural crest; r } roof plate; s, sulcus of Monro; v, ventral fissure. 



while the lumen of the tube, the central canal, is oval in section, its 

 side walls, consisting of embryonic nervous tissue, being thicker than 

 roof or floor. 



From the method of its formation it will be seen that the inner surface of the 

 neural tube is homologous with the outer surface of the general epidermis of the 

 body. The account given above is not exact for cyclostomes, teleosts and some 

 ganoids, where the neural plate forms a keel extending below the surface, in 

 which a central canal appears later, so that the final result is closely similar to 

 the typical condition. 



The Spinal Cord. 



From this simple tube the spinal cord of the adult is developed by 

 several modifications. The cells of the side walls proliferate rapidly, 



