ORIGIN OF THE LATERAL LINE PRIMORDIA 611 



After its appearance it grows rapidly forward and seems to split 

 into the supra-orbital and sub-orbital lines in which later the 

 lateral line organs appear. The lines are quite distinct from each 

 other at their posterior ends soon after their appearance. 



The primordium of the mandibular line appears later than that 

 of the supra- and infra-orbital lines. It lies ventral to the hyoid 

 thickening from which it is completely detached. Its position 

 in a dorso-ventral diameter is on a level with the anterior end 

 of the hyoid thickening. Its late appearance and complete de- 

 tachment from other thickenings in the hyoid region leave no 

 doubt as to its distinct origin. 



The postauditory region 



1. Under the term 'postauditory region' is included all that 

 area whose lateral line organs are supplied by the IX and X nerves. 

 The area supplied by the IX nerve lies at the level of the ear and 

 is not strictly postauditory in position, although supplied by a 

 postauditory nerve. In the postauditory region there is no post- 

 auditory placode or posterior extension of the auditory vesicle 

 and the problem as to the origin of the postauditory lateral lines 

 from the vesicle is easily disposed of. There remains only the 

 problems as to what structures are so situated as to be mistaken 

 for lateral line primordia and the exact time and place of appear- 

 ance of the lateral line primordia. 



2. In the postauditory region occur the same structures with 

 the exception of the auditory placode that were found in the 

 preauditory region. These structures are more numerous on 

 account of the presence of five functional gills; but on the whole 

 are easier of interpretation on account of the fact that the gills 

 become functional, whereas the hyoid gill behaves as if it were 

 going to open but later the endodermic pocket withdi-aws from 

 the ectoderm leaving the epibranchial placode to furnish the only 

 permanent contribution to structures in the head. The post- 

 auditory region is further rendered easier of interpretation on 

 account of the repetition in each gill of similar structures, thus 

 furnishing a basis for comparison of the several gills with each 

 other and with the hyoid gill, and lastly by the absence of a 

 postauditory placode. 



