ORIGIN OF THE LATERAL LINE PRIMORDIA 597 



THE POSTAUDITORY REGION 



Since there is no posterior extension of the auditory vesicle in 

 Lepidosteus from which the lateral line primordia could be de- 

 rived, the problem of the relation of the auditory vesicle to this 

 primordium would seem at first glance simple of solution. Such 

 is not the case. The existence in the preauditory region, of four 

 distinct structures, three of which are concerned in the forma- 

 tion of the gill slit and the epibranchial placode, and their rela- 

 tion to the lateral line primordia and especially the fact that they 

 are so situated that they seem to furnish a continuity in structure 

 between the vesicle and the lines emphasize the necessity of a 

 careful study of the postauditory region, even though the post- 

 auditory placode is absent. It is certainly present in some types 

 and in a position corresponding to the preauditory placode. 

 Whether it is present and atrophies, as in Ameiurus, or is absent, 

 as in Lepidosteus, there is the same need for a careful study of all 

 the structures that might be mistaken for it or might seem to 

 render continuous the vesicle and the primordia of the lines. 



In the postauditory region there are present all of the struct- 

 ures associated with the gill slit that are found anterior to the 

 ear, namely: (a) the thickenings of the epidermis at the points 

 where the endoderm of each gill pocket joins the ectoderm; these 

 usually appear previous to the opening of the gill slit; (b) the 

 epibranchial placode of each gill; these are situated at the poste- 

 rior end of the ectodermic thickening (a) ; (c) The later thickening 

 of the ectoderm arising behind the area of contact of endodermic 

 gill pocket and ectoderm, these appearing as a posterior extension 

 of the gill thickening. This last thickening persists, however 

 after the gill slit has opened and after the epibranchial placode 

 has become detached from its appropriate gill and has joined the 

 viscerial ganglia of the IX and X nerves, (d) In addition to 

 all these structures there is present in the postauditory region, 

 especially in stages preceding the contact of the endodermic gill 

 pockets with the ectoderm, a thickening sometimes quite pro- 

 nounced, at the point where the more or less vertical wall of the 

 body turns laterally to extend over the yolk sac. 



