ORIGIN OF THE LATERAL LINE PRIMORDIA 587 



tory placode and the auditory vesicle (fig. 17). The cavity of the 

 vesicle is 270 ^ posterior to the hyoid contact and the anterior 

 point of detachment is 100 ix beyond the cavity, or 370 ix posterior 

 to the hyoid contact 



In the postauditory region the thickening of the epidermis men- 

 tioned in the description of the 72-hour stage (figs. 24, 25, Ec. 

 Th.l.) has changed slightly. It extends somewhat posterior to 

 the point of contact of the endoderm of the gill-pocket with the 

 ectoderm and toward its posterior end is somewhat broader in 

 its dorso-ventral axis (fig, 27), a condition which will be shown 

 later to be associated with the appearance of a second thicken- 

 ing of the epidermis associated with the second true gill and the 

 first epibranchial placode of this gill. 



THE PREAUDITORY REGION 



The presence of five closely associated structures in the region 

 between the auditory vesicle and the optic vesicle renders difficult 

 the determination of the mode of origin of the sensory lines an- 

 terior to the auditory vesicle. These five structures are the pre- 

 auditory placode (Landacre '10; 'branchial sefise organ' of Wilson 

 and Beard) the ectodermal thickening at the point where the 

 hyoid gill pocket touches the epidermis, the epibranchial placode, 

 the posterior extension of the epibranchial placode and the 

 primordia of the lateral lines. 



The structures named, with the exception of the posterior ex- 

 tension of the epibranchial placode, are shown in figure 2, which 

 represents the 94-hour stage. In this stage the preauditory plac- 

 ode is 70 IX in length and extends from a point 30 tx in front of 

 the anterior limit of the auditory vesicle to a point 40 ^l behind 

 the anterior limit of the vesicle. 



The hyoid ectodermal thickening extends from a point 130 m 

 in front of the anterior boundary of the auditory vesicle and 30 ix 

 behind the posterior limit of the actual contact of the ectoderm 

 and endoderm where the hyoid gill pocket touches the epidermis, 

 to a point 20 /i in front of the anterior limit of the actual contact. 

 The total length of the ectodermal thickening in this stage is 



