614 F. L. LAND ACRE AND A. C. CONGER 



ings themselves. As each gill thickening becomes detached from 

 the more ventral angular thickening, its posterior end extends 

 back of the area of contact of the endodermic gill pocket with the 

 ectoderm and persists after the gill opens. This posterior ex- 

 tension becomes detached from the angular thickening after the 

 anterior end is free from the same thickening, as mentioned above. 

 At the anterior end of this extension or at the posterior border 

 of the gill slit the epibranchial ganglion is detached from the 

 ectoderm, after which the posterior extension gradually disappears 

 The significance of the posterior, extension of the gill thickening 

 is not clear further than that it is out of it that the epibranchial 

 placode forms. The epibranchial placodes and their relation to 

 the visceral ganglion have been fully discussed in a previous 

 paper (Landacre '12). 



6. The post-auditory lateral line primordia appear in three di- 

 visions, the most anterior of which and the first to appear lies 

 in a longitudinal axis on a level with the thickening of the first 

 true gill. In the dorso-ventral axis it lies dorsal to that thicken- 

 ing. It appears almost simultaneously with the appearance of 

 the operculum and its anterior end lies over the anterior attached 

 border of the fold and it extends from this point back over the 

 first gill thickening. This lateral line primordium appears at 

 least twelve hours after the complete detachment of the auditory 

 vesicle from the epidermis. 



Wh.ile in a flat reconstruction, such as figure 4, its relation to 

 the vesicle seems to be close, it is really well detached from the 

 ear on account of the distance between the epidermis and the 

 vesicle. In view of the absence of a postauditory placode, there 

 is no reason, in the opinion of the writers, for relating this primor- 

 dium genetically with that of the auditory vesicle. As men- 

 tioned in the body of the paper, the posterior end of this primor- 

 dium comes quite close to the lateral line ganglion of the IX 

 nerve, suggesting the origin of that ganglion from the lateral 

 line primordium, although our stages are not taken sufficiently 

 close together to settle this question. The lateral line organs 

 which differentiate later in this primordium are supplied by the 

 ramus supratemporalis IX. 



