OKIGIN OF THE LATERAL LINE PRIMORDIA 585 



lines. Therefore, only a cursory examination was made up to 

 the point where well developed primordia of the auditory vesicle 

 and preauditory placode can be recognized. 



The description of the relation of the auditory vesicles to the 

 lateral line primordia begins naturally with the earliest trace of 

 these structures that can be definitely identified. This is, of 

 course, the primordium of the auditory vesicle, since this ap- 

 pears some hours before any trace of .the lateral line primordia 

 can be found. The earliest stages are not figured, since these 

 contain nothing that bears directly on the relation of the two 

 structures in question. 



In a 54-hour stage, a lateral thickening of the ectoderm shows 

 the first evidence of the formation of the auditory vesicle and 

 the preauditory placode. This thickening may be traced for a 

 considerable distance parallel to the infolded neural tube. The 

 60-hour stage and the 66-hour stage show only an increase in 

 development, in that the cells in the lateral thickening exhibit a 

 more marked radial arrangement, more of the cell nuclei come to 

 occupy positions in the distal ends of the cells, and the whole 

 cord, which is the primordium of the auditory vesicle and the 

 preauditory placode, becomes thicker and more prominent. 



A 72-hour stage (figs. 23 to 26) shows, for the first time, the 

 auditory vesicle becoming detached from the ectoderm. The 

 process of detachment begins at the posterior lunit of the vesicle 

 and moves cephalad, the vesicle coming to occupy a position at 

 some distance from the epidermis, but connected with it by 

 strands of drawn-out cells and cytoplasm. It should be noted 

 here that the posterior end of the auditory vesicle is not extended 

 backward into a postauditory placode and that, as stated, when 

 it becomes free from the ectoderm it ends in a rounded knot caudad. 

 This condition will be described in detail in the second part of 

 the paper on the postauditory region. Its bearing on the theory 

 of the origin of the postauditory or body lateral hne is of the 

 greatest importance, since the absence of a posterior extension 

 of the auditory vesicle removes from the field of controversy the 

 question as to the origin of the primordium of the postauditory 

 or body line from the auditory vesicle in Lepidosteus. 



