OEIGIN OF THE LATERAL LINE PRIMORDIA 581 



vagination of the nervous layer of the ectoderm, and forms later a 

 closed vesicle. The lateral line appears first in an embryo in which the 

 auditory organ is a closed vesicle. The four primary lines (supra-or- 

 bital, infra-orbital, hyo-mandibular and body lines) arise independently 

 of one another and of the auditory organ as thickenings of the nervous 

 layer of the ectoderm. 



Landacre ('10) was the first to apply the term ''preauditory 

 placode" to the anterior extension of the auditory vesicle from 

 the point where the vesicle narrows down at its future anterior 

 boundary to the extreme anterior limit of the extension. He 

 states (p. 339) that in Ameiurus for a time the preauditory plac- 

 ode ('branchial sense organ' of Wilson and Beard?) simulates in 

 appearance a lateral line organ, and then later disappears, with- 

 out giving rise to lateral line organs, there being a period of 

 more than twenty-six hours between the disappearance of the 

 preauditory placode and the appearance of the lateral line or- 

 gans. In the region posterior to the auditory vesicle he finds that 

 the postauditory placode loses the characteristic cell arrange- 

 ment, and although the lateral line organs appear along the route 

 traversed by the placode as it moves back, they are not derived 

 from the postauditory placode; further the lateral line organs in 

 Ameiurus appear as individual differentiations of the ectoderm 

 and are not preceded by a continuous line in Ameiurus as in 

 other types described. In a paper two years later ('12) he de- 

 scribes the preauditory placode as disappearing and fails to find 

 any genetic relation between the preauditory placode and the 

 sensory lines. 



From the preceding brief review of the literature it is evident 

 that there are two quite divergent points of view. The first holds 

 that the lateral line primordia, that is, the sensory lines found in 

 approximately the positions in which the supra-orbital, infra- 

 orbital, mandibular and body lines are later situated, are derived 

 from and continuous at first with the epidermal thickening out 

 of which the auditory vesicle forms. 



The second explanation starts with the same primordia, the 

 auditory vesicle and its anterior and posterior prolongations (pre- 

 and postauditory placodes) on the one hand, and on the other 

 the primordia of the lateral lines, namely, the sensory lines; but 



