vii EVOLUTION OF THE NERVOUS SYSTEM 339 



These arguments are directed against the prevailing 

 fashion in comparative anatomy of referring all similarities 

 of form dogmatically to blood-relationship, and of leaving the 

 direct and indirect influences of the external world out of 

 consideration. The view here urged that the nervous system 

 and sense organs have been formed in the multicellular 

 animals from the original body covering, from the epiblast, 

 because the epidermis has naturally from the first been the 

 medium of relation to the outer world, because ' it received 

 impressions and required to be rendered capable of giving 

 the impulse to reaction against those impressions, to defence 

 and attack, this view will be opposed by no embryologist or 

 physiologist. But perhaps the most important support for 

 this view will be afforded by tracing the first appearance of 

 a morphologically distinguishable nervous system in the 

 animal series, as I have traced it in the Ctenophora and 

 Medusse. 



THE ORIGIN OF THE CENTRAL NERVOUS SYSTEM 



Embryology has long since taught us that not only in the 

 Vertebrata, but also in the Arthropoda, Mollusca, and 

 Vermes, the central nervous system is developed from epiblast- 

 cells, which separate from the superficial layer whence they 

 are derived and pass to a deeper position, and has also shown 

 that such animals develop from similar embryos which 

 consist of germinal layers, and which are evidently genetically 

 related together. It has also long been known that lower 

 forms of the Metazoa exist, which remain throughout life 

 essentially in the morphological condition of these embryos, 

 and which although no separate nervous system -can be recog- 

 nised in them, clearly manifest the power of sensation and 

 even of volition. Thus the question necessarily arose whether 

 these powers in such forms do not reside in the ectoderm. 



Accordingly, N. Kleinenberg has explained the cylindrical 



