32G Mr A. Sedgwick, The original function of the canal [Feb. 2G, 



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 obviously 

 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 

 Vertebrates ; 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 re- 

 spiratory 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 improbable 

 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 proceeded from the groove stage, by the 

 conversion of the groove into a canal open at either end. The 

 neural canal therefore owed its origin to the requirements of 

 protection and respiration. When once formed it must have 

 continued to discharge some function, otherwise it would have 

 atrophied, as we know is the habit of useless structures. The 

 function of the canal at this stage of its evolution is the subject of 

 the present discussion. 



The relations of the neural canal at the siphon stage, which is 

 well marked in the development of Ascidians and Amphioxus, are 

 well known to all students of embryology. It is open behind into 

 the hind end of the alimentary canal and in front, in the cephalic 

 region of the body. 



How it acquired its opening into the alimentary canal is 

 perhaps hard to understand. The discussion of this question 

 involves the discussion of a still more difficult question, viz. the 

 relation of the permanent anus to the blastopore ; this I reserve 

 for a subsequent occasion. I may however point out here that 



