586 F. L. LANDACRE AND A. C. CONGER 



The ectodermal thickening of the first true gill is present in 

 this stage. It lies ventro-laterally from the auditory vesicle (figs. 

 24, 25, Ec.Th.l.). Its total length is 150 ^ and it is coextensive 

 with the attachment of the auditory vesicle to the ectoderm. 

 The anterior thii'd of the thickening Ues in the area of contact of 

 the pharyngeal pocket of the first true gill with the ectoderm 

 and two thirds of the thickening extends posterior to the gill 

 contact as a thickened cord in the ectoderm. It was mistaken 

 at first for a postauditory placode, but its later history shows it 

 to be a true gill thickening comparable with the similar thicken- 

 ings on the VII and the other four true gills. 



In this stage the radially arranged cells of the preauditory 

 placode (anterior extension of the auditory vesicle) can be rec- 

 ognized extending forward to within three sections of the posterior 

 limit of the actual contact of ectoderm in the region of the hyoid 

 gill pocket. In the description of later stages we shall use as a 

 land-mark the last section which shows a connection between 

 the ectoderm and endoderm in the region of this gill-pocket, and 

 structures will be located by the number of sections posterior to 

 the point of 'hyoid contact.' 



In the 72-hour stage at a point seven sections (70 ix) posterior 

 to the hyoid contact the preauditory placode resembles the audi- 

 tory vesicle except in size Cephalad of this point it does not 

 resemble the vesicle closely and becomes less prominent towards 

 its anterior end. The radial arrangement of the cells is very 

 pronounced and the placode has become so thick that it projects 

 well beyond the inner surface of the ectoderm. There is no 

 sharp differentiation between the preauditory placode and the 

 auditory vesicle, and one reads back without break in continuity 

 of structure directly into the true vesicle. Twenty-five sections 

 posterior to the hyoid contact, the cavity of the vesicle may be 

 recognized, and eight sections beyond this, or 330 /x posterior to 

 the hyoid contact, the vesicle is detached from the ectoderm. 



In a 76-hour stage, the anterior end of the preauditory placode 

 has moved back one section from its position in the preceding 

 stage, or four sections posterior to the hyoid contact. There is 

 still histologically no sharp differentiation between the preaudi- 



