3? 

 evolution of tliis system is bound up with tliat of the peculiar 

 series of sense-or-^nns which it supplies. Until the evidence 

 is nior-e nearly coiriplete as to the emLryolo^y and affinities of 

 the lateriil line and its nerves as a whole, we really are not 

 warranted in oiaking any positive assertion as to its phyloge- 

 ny. Some evidence as to the possible orit^in of its centre 

 will be ^iven in Subsectioi' 7, 



5. Seurones of the Ventral Ccrnu. 

 Upon reaching' the level of the oblongata, the ventral 

 cornua of the cord beconie broken up as distinct collections of 

 nervous matter. Somatic irotor neurones froD> this source are 

 grouped into the nucleus for the VI nerve, but elsewhere there 

 are only isolated individuals lyin^ between the fibres of the 

 formatio reticularis. In an earlier paper touching this sub- 

 ject ('9?b), I left the inter[,retation of these scattered neu- 

 rones undecided. It is no/f certain that they correspond to 

 the coiiimissural cells and the tract-cells ( vorderstrangzellen ), 

 respectively, which vonLenhossek ('94) has described from the 

 spinal cord of the selachian. 



a. Tract-Keurcnes. — The ventral tract-neurones are scat- 

 tered over the mid-ventral field of the oblongata on each side 

 of the raphe ventral to the dorsal longitudinal bundles (Fig.?, 

 t.n.). They are to be readily distinguished from all other 



