NERVOUS SYSTEM 



143 



this way a brain has been specialized apart from the rest of the 

 nervous system. 



With the appearance of metamerism in the mesothelium and the 

 development of muscles from the myotomes there results a serial 

 repetition of motor nerves going to these, since each muscle must have 

 its own nerve supply, while sensory nerves are the result of the sink- 

 ing of the neural plate to a deeper position, as the sensory organs 

 must be largely in the skin. 



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

 brates is not yet satisfactorily explained. The fact that in Atnphioxus 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. 12) 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 neural (or medullary) plate (p. 12) is a thickened portion of 

 the ectoderm which surrounds the line of closure of the blastopore. 



Fig. 151. — Earlier (A) and later (5) transverse sections of Amblystoma embryos, 

 B being also also more posterior, a, archenteron; e, optic pit; en, entoderm; m, meso- 

 thelium (with somatic and splanchnic layers; n, notochordal cells and (5) notochord; nc, 

 neural crest; ng, neural groove; np, neural plate; p, primitive groove. 



It is thinner in the middle line than elsewhere (primitive groove), and 

 at the margins passes rather suddenly into the unmodified ectoderm 

 which is to form the epidermis (fig. 151, ^4). With further develop- 

 ment the edges of the plate are elevated as neural folds (fig, 151, .6) 



