CHAPTER XV. 



NERVOUS SYSTEM. 



Origin of the Nervous System. 



ONE of the most important recent embryological discoveries is 

 the fact that the central nervous system, in all the Metazoa in which 

 it is fully established, is (with a few doubtful exceptions) derived 

 from the primitive epiblast 1 . As we have already seen that the epi- 

 blast represents to a large extent the primitive epidermis, the fact of 

 the nervous system being derived from the epiblast implies that the 

 functions of the central nervous system, which were originally taken 

 by the whole skin, became gradually concentrated in a special part 

 of the skin which was step by step removed from the surface, and 

 has filially become in the higher types a well-defined organ imbedded 

 in the subdermal tissues. 



Before considering in detail the comparative development of the 

 nervous system, it will be convenient shortly to review the present 

 state of our knowledge on the general process of its evolution. 



This process may be studied either embryologically, or by a 

 comparison of the various stages in its evolution preserved in living 

 forms. Both the methods have led to important results. 



The embryological evidence shews that the ganglion-cells of the 

 central part of the nervous system are originally derived from the 

 simple undifferentiated epithelial cells of the surface of the body, 

 while the central nervous system itself has arisen from the concen- 

 tration of such cells in special tracts. In the Chordata at any rate 

 the nerves arise as outgrowths of the central organ. 



Another important fact shewn by embryology is that the central 

 nervous system, and percipient portions of the organs of special sense, 



1 Whether there is any part of it in many types not so derived requires further 

 investigation, now that it has been shewn by the Hertwigs that part of the system 

 develops from the eudoderm in some Coelenterata. 0. Hertwig holds that part of it has 

 a mesoblastic origin in Sagitta, but his observations on this point appear to me very 

 inconclusive. It would be very advantageous to investigate the origin of Auerbach's 

 plexus in Mammalia. 



