THE ANCESTIIY OF THE OHOEDATA. 93 



Amphioxus seems to indicate that the process by which they 

 became so occurred first anteriorly. 



Let us now follow the history of the ventral roots as pre- 

 served to us. In Amphioxus the large nerves or dorsal roots 

 supply the skin and certain sense organs placed among the 

 muscular tissue (Rohon) ; but into each myotome, opposite 

 each dorsal root, runs a bunch of loose nerve-fibres from the 

 cord. This was stated by Rohon, but denied by Balfour. 

 Improved methods of section cutting leave no doubt, however, 

 that Rohon's observation was correct, and, indeed, these fibres 

 may be easily seen. The presence of these bunches of fibres 

 clearly gives us another step in the formation of the '''seg- 

 mented" nervous system. For in the simplest case, that of 

 Balanoglossus, the muscles are not gathered into bunches, and 

 the nerve-fibres likewise are irregular. In Amphioxus the 

 muscles are already gathered into bundles, and the motor 

 nerves follow them in this arrangement, but remain distinct 

 from the dorsal roots. This therefore is a stage towards the 

 gathering of the efierent fibres into a "ventral root;" in Bdel- 

 lostoma this is already done, and though the dorsal roots are 

 already approximately, though not quite opposite each other, 

 yet the ventral roots are not at the same level with them. 

 Besides this, in Lampreys, the anterior and posterior roots are 

 still not united into a common cord, though in Myxine they 

 are thus arranged (Schneider and others). 



In this the nervous systems of Balanoglossus, Amphioxus, 

 Lampreys, and Myxine form a graduated series leading up to 

 the condition found in higher Vertebrates, showing the evolu- 

 tion of the nervous system of Vertebrata from a solid cord in 

 the skin to its condition as a closed tube whose walls give off 

 a series of " segmental" nerves arising by roots of difi'erent 

 functions. 



[It will be seen that if this view be accepted it becomes vei'y 

 doubtful whether efforts to analyse the segmentation of the 

 head can lead to any result, seeing that it almost follows that 

 the head was differentiated as such before any complex meta- 

 merisation was present; and, indeed, were it not for theoretical 



