The Origin of the Electric Organs in Astroscopus Guttatus. 153 



STUDY OF AN EMBRYO 20 MILLIMETERS IN LENGTH. 



The head of the 20 mm. embryo was sectioned serially and trans- 

 versely and reconstructed in 1 mm. wax at a magnification of 500. 

 This embryo was so well preserved that a very careful study has been 

 made of it (plates ii and iii). 



The electric area has become definite electric tissue and is now seen 

 for the first time on the obliquus superior muscle, making it evident 

 that the electric tissue is generated from muscles which are innervated 

 by the fourth and sixth nerves as well as from some which are inner- 

 vated by the third nerve. The three rectus muscles which contribute 

 to the electric tissue have a common origin in the subcranial canal, the 

 rectus inferior muscle having a separate origin. The obliquus superior 

 arises at some distance from the three rectus muscles, but is the more 

 dorsal of the two obliquus muscles. The fourth and sixth nerves take 

 no part in the innervation of the adult electric organs. The organs 

 are exposed dorsally, on the other hand, and only those muscles which 

 are nearest the dorsal surface of the head have contributed to their 

 formation, the two inferior muscles on the ventral surface not being 

 concerned. Torpedo is the only other fish known to have the electric 

 organs on the exposed or the dorsal surface, even though it occupies all 

 the space between the dorsal and ventral surfaces. This tendency to 

 develop the electric organs on the exposed or dorsal surface may be 

 compared with the opposite tendency of fishes and other animals to 

 develop their luminous organs on the ventral or protected surface, as 

 shown in Porichthys. This correlation of the protective organ with the 

 exposed surface and the attractive organ with the protected surface 

 may be the result of natural selection, even though Darwin (37) con- 

 sidered electric organs to be one of the greatest difficulties in the way 

 of his theory. 



The electric cells have assumed the form of electroplaxes (plate v, 

 fig. 4). They have enlarged to about six times the diameter of the ordi- 

 nary muscle-cells; and the large and numerous nuclei have rounded up 

 close to the membrane, although a few remain scattered in the cyto- 

 IDlasm. The cytoplasm itself has become peculiarly vacuolated and 

 very definite striations have appeared. In longitudinal section each 

 electroplax shows the ventral end indented into long, finger-like pro- 

 cesses (plate VI, fig. 1). The striations across these processes are 

 continuous, as though the cell had been cut through at this point after 

 the striations had been formed instead of the processes having grown 

 out. The striations are perfectly definite and pass straight across the 

 cell with no indication of the myofibrillation. Long, narrow vacuoles 

 appear at intervals in the cytoplasm of the indented or evaginated end, 

 while the dorsal end of the cytoplasm is filled with vacuoles of a 

 smaller, rounder nature. This indentation is the beginning of the 

 formation of the ventral papillae of the adult electroplax. 



