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



INTRODUCTION 



The lack of agreement among workers as to the relation of the 

 lateral line primordia — that is, the lateral sensory ridges preced- 

 ing the definite lateral line organs — -to the auditory vesicle, is 

 sufficiently marked to require a re-examination of the whole 

 question. Part of this lack of agreement may be due to 

 differences in the types studied, but as a whole it can hardly 

 be explained in this way. 



H. V. Wilson ('91) and a number of other workers trace the 

 primordia of the lateral line organs directly to the auditory ves- 

 icle and describe the earliest stages of these primordia as ante- 

 rior and posterior extensions of the same epidermal thickening 

 that gives rise to the auditory vesicle. On the other hand, Piatt 

 C95-'96), Clapp ('99), Beckwith ('07) and Landacre ('10) find 

 no such connection between the primordia of the sensory lines 

 and the vesicle, but find the primordia arising close to the vesicle 

 but independent of its anterior and posterior extensions. 



Landacre ('10) found in Ameiurus no extended sensory ridges 

 preceding the appearance of definitive lateral line organs, although 

 several organs may arise sometimes close together from a short 

 epidermal thickening. He also traced the degeneration of the 

 anterior and posterior extensions of the auditory vesicle before 

 the lateral line organs appeared. Ameiurus thus furnishes strik- 

 ing evidence for the independent origin of the lateral line organs 

 and auditory vesicle. 



Since, however, in Ameiurus the lateral line organs arise inde- 

 pendently and are not preceded by lateral sensory ridges, it seems 

 desirable to go carefully over the origin of these structures in 

 some type which has well defined sensory lines preceding the ap- 

 pearance of definitive lateral line organs to determine, if possible, 

 the exact relation of the lateral line primordia to the auditory 

 vesicle; and to determine further, if these primordia arise inde- 

 pendently, what embiyological structures are so situated as to 

 lead workers to infer a relationship between them that may not 

 exist. 



