82 MICHIGAN ACADEMY OF SCIENCE. 



OS THE EARLY HISTORY OF THE AUDITORY AND LATERAL 



LINE ORGANS OF AMIA. 



CORA J. BECKWITII, I'KKSKXTED BY JACOB REIGHARD. 



(Abstract.) 



A study was made of the early stages in the development of the aud- 

 itory and lateral line organs of Amia, with a view of determining 

 whether these originate from a common anlage or independently. It 

 was shown: 



1. That the auditory vesicle is formed in the midst of a mass of 

 mesectoderm, differentiated from the adjacent .neural crest. The vesicle 

 appears in surface view to be continuous with this mass of mesenchyme 

 and to form a part of it. Subsequently the mass of mesenchyme divides 

 into two parts corresjjonding to the first two branchial arches and the 

 auditory vesicle lies between these two portions. In surface view the 

 original mass of raesenchvme with the included auditorv vesicle has 

 The appearance of a proliferation or ridge of ectoblast — a common anlage 

 of auditory vesicle and lateral line systems. When this original mass 

 of mesectoderm divides in the manner indicated and its two i)ortions 

 separate from the auditory vesicle, the appearance is as if the com- 

 mon anlage of auditory vesicle and lateral line systems had divided 

 into three portions. Such a division was described by Wilson in the 

 Trout and the three resulting portions were believed by him to give rise 

 To the branchial sense organ, the auditory vesicle and the lateral line 

 anlage respectively. In Amia there is a simulation of the division of 

 such an ectoblast ridge into three portions, but in reality two of these 

 masses are mesenchyme while the third is the auditory vesicle which is 

 from the first indej)endent of this mesenchyme. 



2. The anlage of the lateral line system was found to arise at a much 

 later stage than the auditory vesicle, and quite independently of it in 

 the form of ridges or thickenings of the ectoblast. 



