ORIGIN OF THE LATERAL LINE PRIMORDIA 593 



of 100 fj. in front of the anterior boundary of the auditory vesicle, 

 while in the next stage (94 hours) the preauditory placode ex- 

 tends only 30 ix in front of the anterior limit of the vesicle and at 

 100 hours it is no longer present. 



On the other hand, the posterior extension of the epibranchial 

 placode can first be recognized in the 100-hour stage. From the 

 fact that the posterior extension of the epibranchial placode is 

 continuous with the epibranchial placode, while the preauditory 

 placode has already disappeared and that it exhibits none of the 

 histological characters of the preauditory placode, and that its 

 growth backwards follows the receding of the preauditory plac- 

 ode after a lapse of several hours, we may assume that there 

 was no genetic relation between the preauditory placode and the 

 posterior extension of the epibranchial placode. Previous to the 

 stage when the posterior extension of the epibranchial placode is 

 recognizable (fig. 3), the primordium of the anterior sensory lines 

 is already formed, anterior to and dorsal to the epibranchial 

 placode, as shown in figures 2 and 3. 



e. The primordia of supra-orbital and infra-orbital sensory lines 



In the 94-hour stage (fig. 2) an ectodermal thickening appears 

 in the region just dorsal and anterior to the epibranchial placode. 

 Examination of later stages show this structure to be the pri- 

 mordium of the sensory lines upon which the lateral line organs 

 of the supra-orbital and sub-orbital lines will later be formed. 



As has been previously pointed out, the anterior end of the 

 preauditory placode degenerates, so that, in the 94-hour stage 

 (fig. 2), the radial arrangement of its cells can be recognized for 

 30 M only in front of the anterior limit of the auditory vesicle. 

 Between the anterior end of the preauditory placode and the 

 posterior limit of the epibranchial placode, which can first be 

 recognized in this stage, a distance of 70 /j, intervenes. Since 

 the preauditory placode lies at a level slightly dorsal to that 

 occupied by the epibranchial placode, and in addition certain 

 histological differences may be noted, the two structures are 

 easily distinguished. 



