DEVELOPMENT OP PETBOMYZON FLUVIATILIS. 209 



There is no trace of the ramus lateralis of the vagus even in 

 my oldest larvae. 



The ganglion on the ninth nerve lies in front of the first 

 myomere, between that and the ear, whilst that of the vagus 

 lies between the first and second. The first dorsal root of the 

 spinal nerves with its ganglion lies between the third and 

 fourth myomere. Behind this there is a dorsal ganglion lying 

 opposite each myotome. 



Sagemehl (17) has described very correctly the origin of 

 the spinal nerves. The dorsal roots with their ganglia arise 

 from a neural ridge which is at first of the same size all along. 

 From this the ganglia begin to grow out about the eighteenth 

 day, intersegmentally, that is opposite the myotomes. The 

 ganglia are in connection with one another for some time by a 

 longitudinal commissure. This commissure appears to consist 

 of the remains of the neural ridge ; it ultimately disappears, as 

 in Elasmobranchs. The dorsal nerves, after leaving the 

 ganglia, run into the myotomes and eventually, I believe, reach 

 the skin, though on this point I cannot be quite certain. On 

 the other hand the ventral roots consist of nerve-fibres only, 

 and run straight into the myomeres. They appear, according 

 to Sagemehl, very soon after the first appearance of white 

 matter in the chord, and they never have any connection with 

 the dorsal roots. The resemblance between the distribution of 

 the spinal nerves of this larva with those of Amphioxus as 

 described by Rohon is very striking. 



The ear is formed, as Scott has described, from an invagina- 

 tion of the epiblast. This appears very early about the four- 

 teenth day. It soon deepens and becomes completely shut ofl', 

 consisting then of an oval vesicle with a dorsally placed stalk, 

 the recessus labyrinthi. This last is the remains of the duct 

 leading to the exterior. The ear is in the same condition in 

 my oldest larvse. No signs of the semicircular canals have 

 appeared. The epithelium lining the vesicle is high and 

 columnar ; about the twenty-second day certain patches of the 

 epithelium become higher than the others and the cells develope 

 each a very large cilium which projects into the cavity and 



