1010 



(Van Wijhe, 1882, p. 41), since it originates from the lateral plate. 

 The vagus is considered by Hatschkk (1892, p. 152) to be a 

 primarily single nerve belonging to the second post-otic somite but 

 w^hich has collected in its ramus lateralis the lateral dorsal branches 

 of all the spinal nerves behind it and in the same way in its ramus 



-P^ ^ /^ y — ^^ 



Fig. 1. The Hypoglossus of Petromyzon, after Neal (1897) 



1 primarily epibranchial ventral roots. 



2 secondarily „ ,, „ (hypoglossus). 



branchio-intestinalis (with the rami post- and praetrematici), the 

 corresponding ventral branches of as many subsequent nerves as it 

 supplies more than one gill-slit ("partial polymerism" of the vagus). 

 The ramus branchio-intestinalis, which may be compared to the 

 epibranchial plexus of Amphioxiis, no doubt, like the latter also 

 owes its origin to the process of dispersion of the gill-slits. 



For the first time we meet in Petromyzon the beginning of a 

 cartilaginous skull, comparable to the head cartilage in Cephalopods 

 and arising, like the latter, round the central part of the nervous 

 system and the main sense-organs. It contains, besides the prostomium 

 (which Hatschkk does not distinguish from the first segment, design- 

 ating it together with the latter as the acromerite, 1910), only two 

 segments (Hatschkk, 1892, p. 159), as the skull ends with the 

 auditory capsule. The roof remains for the greater part membranous 

 and membranous walls reach from the auditory capsules to 

 the first neural arch, situated between the third and the fourth post- 

 otic myotome. Through this membranous wall the glossopharyngeus 

 and the vagus pass. 



Attention umst be drawn to the fact that the hypoglossus lies far 

 behind the skull and also far behind the vagus. It does not 

 appear from any publications on the subject that the spinal ganglia 

 following behind the vagus are, under the influence of the latter, 

 less developed than those situated more backwards, as may be 

 noted regularly in Gnathostomes. 



In many respects the Amphibians more closely resemble the 

 Cyclostomes than the Selachians which, though exhibiting several 



