THE NERVOUS SYSTEM OF AMPHIBIA 291 



1. Lateral line primordia, placodes and ectodermal thickenings 



In the use of the terms 'lateral Hne primordia,' 'placodes' and 

 'ectodermal thickenings,' I am following Landacre and Conger 

 in their work upon Ameiurus and Lepidosteus (Landacre, '10, 

 '12; Landacre and Conger, '13). I wish, however, at the outset 

 of this discussion, to emphasize the fact that mj- series of Ambly- 

 stoma embryos is not adapted to a critical study of the histo- 

 genesis of these structures, for my specimens have been selected 

 at relatively long intervals according to physiological stages and 

 with reference to a problem that is not primarily one of histo- 

 genesis. It seems obvious, nevertheless, that there is a striking 

 resemblance between Amblystoma and Lepidosteus (Landacre 

 and Conger, '13) in the early processes of differentiation of sense 

 organs and nerves. 



While I have not undertaken to determine to what extent epi- 

 branchial placodes contribute to the formation of the various 

 cranial ganglia, it seems certain that the visceral ganglia of the 

 facial, glossopharyngeal and vagus nerves all receive masses of 

 cells from this source, particularly (hu'ing the middle and latter 

 part of the period covered by my studies. This tardy differen- 

 tiation of the visceral as compared with the somatic sensory 

 ganglia is correlated with a late entrance of the visceral sensory 

 field into the behavior, and is particularlj' noteworthy in connec- 

 tion with Landacre's observation that the relatively late differ- 

 entiation of the ejiibranchial i^lacodes in Lepidosteus as com- 

 pared with Ameiurus is correlated with a late development of 

 the taste buds in the former as compared with the latter, accoitl- 

 ing to particular growth periods. 



No lateral line placodes have been recognized in Amblystoma 

 of m}' series, unless the ectodermal thickening which joins the 

 auditory vesicle to the preauditory lateral line primordium be 

 regarded as a vestige of a dorso-lateral placode. In section this 

 thickening of the skin has the appearance of that which is formed 

 in the early flexure stage over the ophthalmic ganglion where 

 the latter has just become detached from the ectoderm. The 

 significance of this attachment of the ophthalmic ganglion to 



