578 F. L. LANDACRE AND A. C. CONGER 



other epidermal thickenings that might be confused with those 

 concerned in the formation of the lateral line oi'gans. It is quite 

 evident that no definite conclusion can be reached until we 

 know exactly what is to be included, or rather what is to be ex- 

 cluded, in the genesis of this system. This means that we must 

 carefully exclude those placodes concerned in the formation of 

 visceral ganglia, the epibranchial placodes, and probably those 

 concerned in the formation of lateralis ganglia as distinct from 

 lateral line organs, one or both of which these authors included 

 in the formation of the lateral line primordia. As the text of 

 this paper will show, there are still other epidermal thickenings 

 which may be easily taken for lateral line primordia but which 

 do not belong with this system at all. 



Ayers ('92) in commenting on the conclusions of Beard men- 

 tioned above, makes the following statement: ''As Froriep has 

 shown the ectodermal thickenings which Beard described as 

 having given rise to the lateral line organs have in fact another 

 fate. The genuine lateral line organs escaped Beard's attention." 

 Locy ('95) states that the branchial sense organs of Beard and 

 Froriep are not lateral line organs. Cole ('98, p. 151), in his 

 work on Gadus, follows Ayers' criticism of the conclusions of 

 Beard (as cited above) with the following remark: ''It is possible 

 that his (Ayers) latter statement is to some extent sufficiently 

 near the truth to require a re-opening of the whole question of 

 the 'branchial' or 'epibranchial' sense organs." 



Among the papers taking up specifically the relations of the 

 lateral line primordia to the ear must be mentioned those of 

 Mitrophanow and Wilson, Mitrophanow ('91) described a com- 

 mon primordium for the lateral line system and the auditory 

 vesicle. In his observations on the lateral line system of elas- 

 mobranchs ('93), he repeats his first statement, and says that the 

 auditory pits and the sense organs of the lateral line system have 

 a common origin in a thickened band of epithelium. The audi- 

 tory vesicle separates from the sensory band, which is thus cut 

 into two portions ; the anterior portion, lying in the region of the 

 fifth nerve and corresponding to Beard's "branchial sense-organ," 

 gives rise to the supra- and infra-orbital lines, while the posterior 



