1883.] Air A. Sedgwick, The nervous system of Vertebrata. 325 



February 26, 1883. 



Mr Glaisher, President, in the Chair. 



The following were balloted for and duly elected Fellows 

 of the Society : 



Eev. R. Appleton, M.A. Trinity College. 

 Mr R. F. Scott, M.A. St John's College. 

 Mr M. J. M. Hill, M.A. St Peter's College. 

 Mr W. A. Bond, M.A. St John's College. 

 Mr J. Larmor, B.A. St John's College. 



The following communications were made to the Society : 



(1) The original function of the canal of the central nervous 

 system of Vertebrata. By A. Sedgwick, M.A. 



The central nervous system of all known animals with certain 

 doubtful exceptions, arises from the epiblast. The region of the 

 epiblast from which it arises may either persist in the adult as 

 part of the superficial epidermis, or it may be pushed in so as 

 to give rise to a tube, from the walls of which the central nervous 

 system is developed. The last mentioned method is characteristic 

 of the Vertebrata. The walls of this tube become differentiated 

 into a superficial epithelial layer lining it, and an external mass of 

 nervous matter. The tube persists as the canal of the nervous 

 system ; the epithelium lining it becomes the ciliated epithelium 

 of this canal, which therefore corresponds to the external epithelium 

 of the body-wall. 



I may here draw attention to the fact that the vertebrate 

 stock must have separated from that of other animals before the 

 nervous system was separated from the external epithelium of the 

 body ; that in fact the vertebrate nervous system never is separated 

 by any ingrowth of mesoblast from the superficial epiblast from 

 which it arose ; as is the case in all but the most primitive of the 

 Invertebrata. This superficial epiblast in Vertebrata is involuted 

 and gives rise to the ciliated epithelium just mentioned of the 

 central canal. Three stages may be distinguished in the develop- 

 ment of this canal, and I suppose that all three have had a 

 functional counterpart in the evolution of the organ. 



In the first stage a groove extended along the whole length 

 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 may be 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. 



