572 



took its origin in connexion with a dorsal nerve which was purely 

 motor, and it is from the tube that the motor nerves all originate in 

 the Craniates. The sensory nerves are derived separately from a re- 

 gion just outside the neural groove from the gangha of sensory or- 

 gans segmentally repeated. These ganglia were next connected by 

 longitudinal commissures, thus forming the primitive lateral line. The 

 ganglia were also connected with the neural tube and with the gill 

 clefts, where the branchial system of sense organs was formed. The 

 separation of the lateral line posteriorly from the brain and spinal 



^1 I Fig. 7. Schematic diagram of primitive Craniate nervous system. Tlie brain seg- 

 ments and nerves are numbered. The mandibular, hyoid, and five branchial arches are 

 shaded, the suppressed arches are shown in outline. 



cord left nothing behind evidently but motor connectives, and con- 

 sequently the spinal ganglia have apparently had a later phylogenetic 

 origin to furnish the sensory fibres of that region of the body. An- 

 teriorly the mesencephalic flexure produced as has already been stated 

 great changes and set free sensory ganglia which as has been sug- 

 gested have also been utilized in connexion with the VII nerve in the 

 lateral line system. 



The auditory organ, which is a specially developed lateral sense 

 organ is developed between the anterior and posterior portions of the 

 lateral line immediately behind the abducent somite, and it is inter- 

 esting at least to note that the brain segment and the nerve is the 

 middle one of the visceral and epibranchial series. 



It may be suggested also that the limits of the brain have been 

 determined by the gills. At all events, notwithstanding the con- 

 ditions in Bdellostoma, these must have had an important influence 

 in determining the region to which it was necessary to give rigidity. 



I wish to take this opportunity of thanking Mr. L. S. Wood for 

 making the microphotographs which accompany this paper. 



Armstrong College, Newcastle-upon-Tyne, 19th March 1910. 



