ORIGIN OF THE LATERAL LINE PRIMORDIA 589 



In a 76-hour stage (fig. 17) the preauditory placode appears as a 

 cord of cells which may be traced cephalad from the anterior end 

 of the auditory vesicle to a point 40 ix posterior to the contact of 

 the ectoderm and endoderm at the hyoid gill pocket. 



Six hours later, the 82-hour stage, shows a slight modification 

 of the anterior end of the preauditory placode in the partial loss 

 of the radial arrangement of the cells. The placode (fig. 19) is 

 in contact with a posterior extension of the ectodermal thickening 

 extending caudad from the point where the hyoid gill pocket 

 comes into contact with* the epidermis, but in this stage 50 ix in- 

 tervene between the anterior end of the preauditory placode and 

 the extreme posterior limit of the actual contact of ectoderm and 

 endoderm. In addition there is the histological difference between 

 the preauditory placode which retains some indication of the 

 radial arrangement of its cells and the hyoid ectodermal thicken- 

 ing which does not show any indication of such arrangement. In 

 th 88-hour stage (figs. 1 and 22) the preauditory placode is further 

 shortened, the cells at the anterior end losing their characteristic 

 radial arrangement so that it can be traced forward now only 

 to a point 70 n posterior to the actual contact. It should be 

 noted, however, that there is a thickening of the epidermis ex- 

 tending from the posterior limit of the hyoid contact back to the 

 anterior end of the preauditory placode, but it does not show a 

 radial arrangement of cells, as does the preauditory placode. 



The last trace of the preauditory placode is found in the 94-hour 

 stage (fig. 2). In this stage the placode has lost its connection 

 with the auditory vesicle, but overlaps the vesicle, extending 30 tx 

 anterior and 40 // posterior to the anterior boundary of the vesicle. 

 The connection with the ectodermal thickening mentioned above 

 has also been lost by the disappearance of the anterior end of the 

 preauditory placode, and 30 At intervene now between the anterior 

 end of the placode and the posterior hmit of the ectodermal 

 thickening, which extends 90 /x behind the actual contact of the 

 endoderm of the gill pocket with the general ectoderm. The dis- 

 tance between the preauditory placode and the actual contact 

 of the ectoderm and endoderm is, therefore, 120 ^ in this stage. 



