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



190 IX, while the actucal contact is 140 m in length (the contact is 

 shown in figure 2 by an unshaded line). 



The epibranchial placode, which is first recognizable in this 

 stage, occupies the region immediately behind the posterior limit 

 of the thickening of the ectoderm in the region where the hyoid 

 gill pocket approaches the epidermis. The epibranchial placode 

 is 30 M in length in this stage and its long axis lies in a dorso- 

 ventral plane. 



The posterior extension of the epibranchical placode, which 

 does not appear in the 94-hour stage, will occupy the region just 

 posterior to the epibranchial placode (fig. 3), and will he at a 

 lower level than that occupied by the preauditory placode. Dur- 

 ing the maximum development of the posterior extension it may 

 be recognized for a distance of four or five sections behind the 

 epibranchial placode. 



Since these four structures play so important a part in the 

 study of the pre-auditory region, we shall take them up in order, 

 and trace the history of each up to a stage when well-developed 

 sensory lines can be recognized. 



' a. Preauditory placode 



Of the structures named above, the first two, the preauditory 

 placode and the ectodermal thickening, appear at very early 

 stages and it would be difficult to state definitely whether one pre- 

 cedes the other or not. The mode' of development of the early 

 stages of the auditory vesicle and the preauditory placode, as 

 observed by the writers, agrees in general with the account by 

 Wilson ('91) in Serranus. In very early stages a thickened cord 

 of cells may be found lying along the neural cord and especially 

 pronounced in the region marked later by the appearance of the 

 auditory vesicle. The central portion invaginates, becomes de- 

 tached, and forms the auditory vesicle. The anterior portion, 

 which is the primodium of the preauditory placode, persists after 

 the auditory vesicle has moved in from the epidermis and assumed 

 its rounded form. The cells of the placode show its relationship 

 to the auditory vesicle by exhibiting the same radial arrange- 

 ment, which is so marked a character of the auditory vesicle. 



