CANAL OP CENTRAL NERVOUS SYSTEM OF VERTEBRATA. IGi 



of the middle dorsal (or ventral?) line of the body, the nervous 

 system being placed in the deeper layers of the epidermis of 

 this groove. This stage I propose to call the groove stage. 



In the second stage^ which maybe called the siphon stage, 

 the groove had become converted into a canal, open in front at 

 or near the anterior end of the body, and open behind close to 

 the anus. 



In the third stage the canal has completely closed; this is 

 the present stage. 



There have been doubtless many other stages, each with its 

 special functional importance in the evolution of the neural 

 canal; but those which I have just mentioned are most ob- 

 viously suggested by the ontogeny of this organ. 



Of the existence of the groove stage there can, I think, be no 

 doubt. It has left its mark in the medullary groove of Verte- 

 brates; and indications of the occurrence of a similar structure 

 are to be found in the embryos of Annelida and Arthropoda. 

 The ventral groove of Neomenia and Proneomenia is probably 

 the same structure persisting in the adult. 



This groove was probably richly ciliated. Once established 

 it soon became deeper. 



The function of the groove was in my opinion partly respi- 

 ratory and partly protective. As the nervous system increased 

 in thickness, the deeper parts of it became so far removed from 

 the surface that the supply of oxygen which reached them was 

 inadequate. This would be remedied by an increase of the 

 surface over which the nervous system was spread, and this 

 increase might be produced by enlarging the superficial area 

 over which the central nervous system extended. It is im- 

 probable that this should happen, because the tendency of 

 evolution seems to have been to localise the central nervous 

 system, probably for the sake of its greater security. The 

 increase of surface required seems to have been obtained by 

 the development of this groove, and the deeper the groove 

 became the more completely would it serve its double function. 

 Thus the way was prepared for the siphon stage which pro- 

 ceeded from the groove stage by the conversion of the groove 



