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 epiblast 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 gradu- 

 ally concentrated in a special part of the skin which was step by 

 step removed from the surface, and has finally 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. 



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 endoderm in some Coelenterata. O. 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 

 . \ucrl >ach's plexus in Mammalia. 



