388 F. L. LANDACRE 



to the Xth, the relative amount cannot be determined and the two 

 components cannot be separated. The visceral ganglia of the 

 second and third gill arches are apparently like the IXth, pure 

 placodal ganglia with possibly a small contingent of lateral mass 

 cells, while the remainder of the tenth is composed of a few placo- 

 dal cells derived from the small fifth and sixth epibranchial plac- 

 odes united to a large lateral mass contingent. 



15. The ganglia described in this paper as general cutaneous, 

 acustico-lateralis, and visceral, have been followed to a late stage 

 and shown to be the ganglia described by the authors under 

 these names. The evidence from Ameiurus is little short of a 

 demonstration that there are separate origins for the general 

 visceral and for the special visceral systems, the former coming 

 from the ventral portion of the lateral mass, or, in other types, 

 from the neural crest, and the latter from the epibranchial placodes. 

 For the acustico-lateralis system, there are two sources of origin. 

 The lateralis Vllth comes entirely from the lateral mass and in 

 other types apparently from the neural crest. The auditory and 

 the lateralis IXth come chiefly from the auditory vesicle but may 

 contain lateral mass cells, while the lateralis Xth comes entirely 

 from the postauditory placode. The description of the acustico- 

 lateralis system as a special cutaneous system finds a strong sup- 

 port in embryological evidence. The lateralis Vllth ganglia rep- 

 resent an intermediate stage between the general cutaneous gan- 

 glia, and the acustico-lateralis ganglia of the IXth and Xth nerves, 

 resembling the former in mode of origin and the latter in structure 

 and function. The latter are derived from the dorso-lateral plac- 

 odes represented by the auditory vesicle and postauditory placodes. 

 The general cutaneous ganglia of the Vth and Xth come from the 

 dorsal portion of the lateral mass also, or neural crest of other 

 types. 



