CRANIAL GANGLIA OF AMBLYSTOMA 487 



vagi nerve leaves the ganglion, fibers are continued ventrally, 

 as on the normal side, to form the visceral trunk of the vagus. 

 A few motor fibers are given off along its path, and when followed 

 farther ventrally it is finally lost near the wall of the pharynx. 

 No ramus intestinalis from the visceral trunk can be determined. 

 In the dorsal portion behind the ear a number of cutaneous fibers 

 may be seen, along with the lateralis fibers, to innervate the skin. 

 The visceral portion of IX may be seen at the lower border of the 

 ear coming off from the root of IX which is at a lower level than 

 on the normal side (fig. 90). The visceral ganglion is small and 

 gives off no trunk to the first branchial arch, but ventral to the 

 ear capsule it sends out a nerve which passes some distance along 

 the median border of the internal ceratohyoid muscle, where it is 

 followed ventrally until its fibers are lost against the wall of the 

 pharynx. The only visceral fibers that can be identified on the 

 operated side appear to be of the special visceral system. The 

 other fibers which appear in the IX and X ganglionic complex on 

 the operated side are of the lateralis and general cutaneous sys- 

 tems. In the ventral positions of the third and fourth branchial 

 arches only very -small rudiments of cartilages appear, which 

 shows that very few crest cells remained which could have con- 

 tributed to the visceral ganglia. 



It appears from the study of the specimens described that the 

 only contribution of the crest cells to the ganglionic complex of 

 IX and X is to the general visceral component. 



DISCUSSION 



It has been shown in the study of early stages of Amblystoma 

 embryos that extensive contributions from the lateral ectoderm 

 take part in the formation of cranial ganglia, and the experi- 

 mental analysis of the problem has shown how small a part the 

 crest cells play in the formation of these ganglia. The facts 

 which the experiments present lead to the conclusion that the 

 general cutaneous system is derived entirely from placodes. In 

 the trigeminal region there are two definite placodes concerned 

 with the formation of the V; that in the case of the ophthalmic 

 division is the larger, while that of the gasserian is the smaller and 



