THE ANCESTRY OF THE CHORDATA. 561 
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, rnns 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 efferent 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 different 
functions. 
[It will be seen that if this view be accepted it becomes very 
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 
