150 Papers from the Department of Marine Biology. 



towings by the steamer Fish Hawk and the schooner Grampus were 

 examined, and a few embryos of different sizes were found, from which 

 the following results have been obtained. Recognition and thanks are 

 also due to Dr. A. G. Mayer and his associates in the Carnegie Insti- 

 tution of Washington who have assisted in procuring these specimens. 

 In a few cases it was difficult to determine whether the embryos were 

 Astroscopus larvae or those of a nearly alhed form, Kathetostomata, which 

 lives in deep water off the Atlantic coast; but since the Sidnlt Kathetos- 

 to7nata has been studied and found to possess no electric organ, there 

 remains no doubt that these embryos which possess the first rudiments 

 of the electric organ are examples of Astroscopus. The embryos were 

 all photographed before sectioning to aid in the identification of the 

 species, but since Astroscopus guttatus and Astroscopus y-grcecum have 

 very similar electric organs in the adult, it matters very little for this 

 work whether the embryos are all of one species or not. Most of 

 them have been identified as Astroscopus guttatus. 



OBSERVATIONS. 



The sectioned heads of these embryos have been handed over to 

 me by Professor Dahlgren for the purpose of tracing the various steps 

 in the development of this curious electric organ and to clear up such 

 points concerning its innervation and blood supply as the material 

 would allow. Some attention will also be given to the change in his- 

 tological structures of the electric tissue during its development from 

 the muscle tissues. 



It quickty became evident that the electric tissue in the young em- 

 bryos was being formed by the metamorphosing of certain parts of 

 some of the eye-muscle, a phenomenon not hitherto met with and there- 

 fore of special interest. 



STUDY OF AN EMBRYO 9.5 MILLIMETERS IN LENGTH. 



The head of this embryo was sectioned serially and transversely and 

 reconstructed with 2 mm. wax at a magnification of 500 (plate i). 

 Some of the eye-muscles of this little embryo are already beginning to 

 show a change, although no definite electric tissue has yet formed. 

 The rectus externus, rectus internus, and rectus superior muscles show 

 a definite darkened area on the side away from the eye-ball, which, upon 

 examination, proves to be composed of cells smaller than the normal 

 muscle- cells. Both the nuclei and the cytoplasm of these cells take the 

 stain much more densely than the other cells, but this is the only sign 

 of any modification of the cell structure (plate v, fig. 2). 



Sections of a 4 mm. embryo were also studied with regard to this 

 particular point, although the specimen was too poorly preserved for 

 any more careful investigation. In this embryo areas corresponding 



