No. 520] ELECTRICITY TISSUES IN FISHES 



in:. 



Before explaining the histogenesis of the electric tissue 

 in this teleost fish a brief resume of what is known of the 

 development of the two elasmohranoh types of electric 

 tissue should he given. 



In the young- embryos of Raja, Ewart and Engelmann 

 found that the position of the future electric organ was 

 occupied by well-developed muscle in no way different 

 from the other muscle tissue of the trunk. In the half- 

 developed fish the transformation of certain of these sin- 

 gle muscle fibers into single electroplaxes was observed 

 to begin, at a later period in those skates which had the 

 simpler types of tissue, at an earlier time in those which 

 had the more highly developed types. This change in the 

 muscle fiber consisted of a widening of its anterior end, 

 which finally resulted in the formation of a flat plate lying 

 at right angles to the position of the former muscle fiber. 

 The posterior end of the fiber degenerated, forming in 

 some cases a useless tail-like appendage in other cases 

 atrophying altogether. 



ioi^curve^ formed the thick " striated " 



layer of the electroplax, while the larger part of the cyto- 

 plasm formed a flat anterior layer or "electric layer" as 

 well as a thick covering, the nutritive layer, on the pos- 

 terior surface. This posterior layer was produced into 

 more or less developed papillae. The nuclei multiplied 

 by amitotic division and were segregated into the ante- 

 rior and posterior layers those in the anterior or electric 

 layer forming a very regular layer themselves. The 

 nerve supply, consisting of several medullated fibers, ap- 

 proached the anterior or electric surface and, dividing 

 into very many fine naked branches, terminated in as 

 many disc-shaped plates in this surface. ^ 



one muscle cell. But there are several important differ- 

 ences. The adult organ is a far more highly specialized 

 structure in Torpedo than in Raja and the muscle cell is 



