ORIGIN OF THE LATERAL LINE PRIMORDIA 599 



relation cannot be stated more definitely on account of the dif- 

 ficulty of determing the exact anterior end of the vesicle owing to 

 the fact that it passes forward at this stage gradually into the 

 preauditory placode. In the last five sections of the area of con- 

 tact of the phryngeal endoderm there appears a thickening of the 

 epidermis at the point of contact. Anterior to this thickening the 

 ectoderm is thin (fig. 23). The appearance of the thickening is 

 shown in figure 24. It lies ventro-laterally from the auditory 

 vesicle and the epidermis is thinner between the two structures 

 than it is at the point of contact of the gill pocket with the ecto- 

 derm. Figure 24 is taken one section anterior to the point where 

 the gill pocket is no longer in contact with the ectoderm. This 

 thickening extends 100 ^t back of this point, so that of its total 

 extent one-third is co-extensive with the posterior portion of the 

 area of contact of the pharyngeal pocket of the first true gill and 

 two-thirds lies posterior to that contact. Figure 25 shows it 50 /x 

 further back than figure 24; from this point it diminishes gradu- 

 ally and its posterior end can be located with difficulty. It 

 certainly does not reach at this stage the level of the posterior 

 end of the auditory vesicle (fig. 26), but does reach approximately 

 the point at which the auditory vesicle becomes detached from 

 the epidermis. 



This thickening can be characterized briefly as an ectodermal 

 thickening lying ventro-laterally from the auditory vesicle, the 

 anterior third of which is co-extensive with the posterior end of 

 the gill pocket contact, the posterior two-thirds extending back- 

 ward in the ectoderm behind the gill pocket contact, and further 

 the whole length of the thickening is approximately co-extensive 

 with the area over which the auditory vesicle is still in contact 

 with the ectoderm at this stage. Its detachment from the audi- 

 tory vesicle, its more ventro-lateral position and its evident con- 

 nection with the gill pocket preclude referring it in any way to 

 the ear or lateral line system. Its later history shows it to be 

 the primordium out of which the epibranchial placode forms as 

 clearly as its early history shows it to be due in some measure 

 to the existence of the forming gill slit. 



