198 THE SPINAL NERVES OF AMFHIOXUS. 



from him as to the conclusions to be drawn from those facts. 

 He does hot, for two reasons, believe that two nerves of Amphi- 

 oxus can be equivalent to a single nerve in higher vertebrates : 

 (i) Because he finds no connecting branch between two suc- 

 ceeding nerves, and no trace of an anastomosis. (2) Because 

 he finds that each nerve in Amphioxus supplies a complete 

 myotome, and he considers it inadmissible to regard the nerves, 

 which in Amphioxus together supply two myojomes, as equiva- 

 lent to those which in higher vertebrates supply a single myo- 

 tome only. 



Although the agreement as to facts between Langerhans 

 and Stieda is apparently a complete one, yet a critical exami- 

 nation of the statements of these two authors proves that their 

 results, on 'one important point at least, are absolutely contra- 

 dictory. Stieda, PI. III. fig. 19, represents a longitudinal and 

 horizontal section through the spinal cord which exhibits the 

 nerves arising alternately on the two sides, and represents each 

 myotome supplied by one nerve. In his explanation of the 

 figure he expressly states that the nerves of one plane only (i.e. 

 only those with dorsal or only those with ventral roots) are 

 represented ; so that if all the nerves which issue from the 

 spinal cord had been represented double the number figured 

 must have been present. But since each myotome is sup- 

 plied by one nerve in the figure, if all the nerves present 

 were represented, each myotome would be supplied by two 

 nerves. 



Since Langerhans most emphatically states that only one 

 nerve is present for eacJi myotome, it necessarily follows that 

 he or Stieda has made an important error ; and it is not too 

 much to say that this error is more than sufficient to counter- 

 balance the value of Langerhans' evidence as a confirmation of 

 Stieda's statements. 



I commenced my investigations by completely isolating 

 the nervous system of Amphioxus by maceration in nitric acid 

 according to the method recommended by Langerhans 1 . On 

 examining specimens so obtained it appeared that, for the 

 greater length of the cord, the nerves arose alternately on the 



