THE EPIBRANCHIAL PLACODES OF SQUALUS 



ACANTHIAS. 



(Fifty-two Figures and Two Tables.) 



Carlos I. Reed, 

 From the Department of Anatomy of The Ohio State University. 



The study of the epibranchial placodes of Squalus Acanthias 

 was undertaken with a view to determining whether this type 

 displays the characteristics with regard to contribution of cells 

 to the visceral ganglia of the gill region by the corresponding 

 placodes, which were described by Landacre ('10) in the catfish 

 and ('12) Lepidosteus osseus. 



In the catfish, this author described a contribution en 

 masse, of placodal cells to Gang. VII, IX and X, by the cor- 

 responding placodes and concluded that the cells from these 

 sources gave rise to gustatory or special visceral fibers. 



In Lepidosteus, it was shown that the method of contribution 

 begins by active proliferation of cells of the ectoderm, thus 

 forming the placode. This process is followed by a contact 

 between the ganglia and the placodes, due to a mesial migration 

 resulting in the fusion of the placodal cells with the general 

 visceral components of VII, IX and X. In Rana, Landacre 

 and McLellan ('12) were unable to distinguish a definite gus- 

 tatory division in the ganglia of the 8 mm. larva probably on 

 account of the more rapid development in this form; but these 

 authors describe the behavior of the epibranchial placodes as 

 similar to that observed in Lepidosteus and Ameiurus, and also 

 describe well defined placodes in the stages earlier than the 

 8 mm. larva. 



The material used consisted of a 20 mm. shark embryo cut 

 into sections 10 microns thick, stained with Delafield's hema- 

 toxylin and counter-stained with Orange G. An 18 mm. 

 embryo, subjected to the same technic was used for comparison. 

 All drawings were made with a camera lucida in magnifications 

 of 50x and 620x respectively, and reduced to one-third the 

 original size in reproduction. 



Owing to the difficulty of securing successive stages of 

 Squalus at close intervals, this study is based upon a comparison 

 of all the epibranchial placodes in one specimen. The placodes 



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