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



At this point the ectoderm is materially thickened especially 

 preceding the appearance of the thickening of the epidermis a*t 

 the actual point of contact of ectoderm and endoderm of the gill 

 pocket. In fact, the gill pocket thickening at its posterior end 

 is usually continuous with the more ventral thickening. Whether 

 this primitive thickening at the point where the vertical body wall 

 turns at nearly right angles to join the wall of the yolk sac is to 

 be thought of as a precursor of the gill pocket thickening or as a 

 strengthened area in the wall, the writers are unprepared to 

 say. It is of special interest at this time on account of the ease 

 with which it might be taken for a lateral line primordium. In 

 fact, both writers did mistake it for a postauditory placode, and 

 at a later stage for the prunordium of the body line. It was only 

 after the true body line primordium had been followed to its 

 earliest stages, and the earliest stages of the gill pocket thicken- 

 ings of the epidermis had been followed, that it was discovered 

 that it had nothing whatever to do with the lateral lines but was 

 intimately associated with the gill slit thickenings, if not the ac- 

 tual precursor of them. In view of these facts, a rather detailed 

 description of all these structures will be given, up to and includ- 

 ing the appearance of the primordium of the postauditory or 

 body sensory line. 



Stage XIII. The description of the postauditory thickenings 

 in the ectoderm will begin with Stage XIIT. At this time the 

 posterior end of the auditory vesicle ends in a rounded knob 

 whose posterior extremity lies nearer the neural canal than it 

 does to the ectoderm. There is no thickening in the epidermis 

 posterior to the hinder end of the vesicle and on a level with it, 

 and there has been none in the stages preceding the one under 

 discussion. There is a thickening lying at a lower level and re- 

 lated in position to the gill pocket in this stage, but it could 

 hardly be mistaken as to its relations. 



The pharyngeal pocket of the first true gill is in contact with 

 the ectoderm throughout twenty-one sections (of 10 ^t each) but 

 has not as yet opened to the exterior. Ahnost the whole area of 

 contact lies anterior to the anterior limit of the auditory vesicle. 

 There is an overlapping of approximately five sections. This 



