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



to the time of detachment. Between the 137-hour stage and the 

 154-hour stage, the placode becomes completely detached and 

 its later history (see Landacre '12) will not be followed here, 

 since lateral lines can be recognized previous to this time. There 

 is, however, little likelihood of confusing the two structures, even 

 though they be close together, on account of the differences of 

 histological structure. 



d. The posterior extension of the epibranchial placode 



The fourth of the structures, whose presence complicates the 

 study of the region anterior to the auditory vesicle, is the ec- 

 todermal thickening which extends caudad from the epibranchial 

 placode, and which we shall call 'the posterior extension of 

 the epibranchial placode,' following the usage of that term by 

 Landacre ('12). 



As has been previously pointed out, the epibranchial placode 

 makes its appearance in the 94-hour stage, at a time when the 

 posterior portion of the preauditory placode still persists from a 

 point 30 fx in front of the anterior boundary of the auditory ves- 

 icle, back to a point 40 n posterior to the anterior boundary of the 

 auditory vesicle. In the 94-hour stage there is an area of 70 n 

 intervening between the posterior limit of the cells of the epi- 

 branchial placode and the radially arranged cells of the preaudi- 

 tory placode. The region between the placodes presents the 

 appearance of normal ectoderm. 



In the 100-hour stage, a structure has appeared in the region 

 from which the preauditory placode has disappeared. This is 

 the posterior extension of the epibranchial placode. There is no 

 difficulty in distinguishing between the preauditory placode and 

 the posterior extension of the epibranchial placode (fig. 8). In 

 the former, the cells are columnar, with a radial arrangement, 

 while in the latter the cells are small in size, irregular in outline, 

 and show no definite arrangement. 



During the period between the 88-hour stage and the 94-hour 

 stage, as mentioned above, the anterior end of the preauditory 

 placode degenerates. In the 88-hour stage the radial arrange- 

 ment of the cells of the placode may be recognized for a distance 



