610 r. L. LANDACRE AND A. C. CONGER 



Its importance can hardly be overestimated on account of its 

 position. It is so located with reference to the preauditory plac- 

 ode and to the primordia of the lateral lines that unless it is fol- 

 lowed with extreme care it seems to render these two structures 

 continuous in the ectoderm and would naturally lead to the theory 

 of the continuity of auditory vesicles and lateral line primordia. 

 Whatever theory is substituted for the theory of continuity of 

 auditory vesicles and lateral lines, in the opinion of the writers 

 the theory of continuity must be abandoned. The most reason- 

 able theory in our opinion is that the sensory areas of the audi- 

 tory vesicle and the lateral line organs are homologous structures 

 with a more or less longitudinal distribution on the body. The 

 auditory sensory areas and the lateral line sensory areas are on a 

 par and the auditory ^^esicle is in no sense the parent of the 

 lateral line primordia. 



5. At an early stage in the history of the posterior extension 

 of the gill thickening the epibranchial placode can be located. 

 Since, however, its history has been followed in detail in an earlier 

 paper (Landacre '12) it will not be described here. In its earlier 

 stages and before detachment of the epibranchial ganglion it 

 forms an intermediate mass in the posterior extension of the gill 

 thickening. 



6. The first primordium of the lateral lines that appears is the 

 common primordium of the supra- and the infraorbital lines. 

 This appears in the same series in which the last trace of the 

 preauditory placode can be found. The preauditory placode 

 previous to the appearance of the lateral line primordium has 

 been undergoing a process of degeneration as described above. 

 When the lateral line primordium first appears it occupies a po- 

 sition exactly dorsal to the epibranchiar placode and in contact 

 with the placode on its ventral surface, but distinguishable from 

 it by its histological character, resembling the preauditory plac- 

 ode in the radial arrangment of its cells, while the epibranchial 

 placode and the whole gill thickening, in fact, have irregularly 

 arranged cells. It is separated from the remnant of the preaudi- 

 tory placode, however, by an area of unmodified ectoderm equal 

 in extent to the to-tal length of the lateral line primordium itself. 



