The Original Function of the Canal of the 

 Central Nervous System of Vertebrata. 



By 



A, Sedg'«Tick, ]U.A. 



The central nervous system of all known animals, -nith 

 certain doubtful exceptions, arises from the epiblast. The 

 region of the epiblast from which it arises may either persist 

 iu 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 auimals 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 development 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 



