The Origin of the Electric Organs in Astroscopus Chittatus. 159 



entering the organs on either side as they go, so that no corner of the 

 four electric organs escapes their reach, no matter how remote. It is 

 possible to trace each nerve to the particular electroplax which it inner- 

 vates, but since all are branches of the same nerve this would have no 

 significance. The nerves enter the electric organs between the electro- 

 plaxes which thej^ enter on the dorsal surface. They can be seen to 

 reach all points on the surface, but the nerve-endings could not be 

 observed because of the unsuitable fixation. 



The fourth nerve can be seen to arise from its separate center in the 

 brain and to have no connection with the electric nerves. 



The circulation has made rapid advance since the 20 mm. stage. 

 Although the blood-vessels in most instances differentiate more rap- 

 idly than nerves, the arteries to the electric organ are not formed until 

 after the innervation has been completed. In the 33 nmi. embryo the 

 branches of the carotid which nourish the eye-muscles have begun to 

 give off small vessels at intervals which enter from the ventral surface. 

 A large branch is given off from the vessel which goes to the rectus 

 superior and another from the vessel which goes to the rectus inferior; 

 other smaller branches are contributed by the vessels which go to the 

 other muscles. A point in this connection is the fact that those mus- 

 cles which have no part either in the formation of the electric organ or 

 in their innervation do contribute toward their nourishment, so that 

 all of the six eye-muscles are in some way involved. 



The rectus inferior muscle has become so reduced that its widest 

 portion is no wider than the nerve which innervates it (plate i, R. inf.). 

 It passes directly thi'ough the electric organs, but at its point of origin 

 on the basiosphenoid bone there are no muscle-fibers at all, a thin con- 

 nective-tissue tendon attaching it to the bone. At a point shghtly 

 dorsal and some distance posterior to its origin muscle-fibers begin to 

 appear and the muscle grows gradually wider. It is small wonder that 

 the eyes of Astroscopus do not have the power of movement of an 

 ordinary fish when the rectus inferior muscle has degenerated to a mere 

 tendon and the rectus superior has been split into several parts by the 

 huge electric nerve. 



STUDY OF AN EMBRYO 45 MILLIMETERS LONG. 



The head of the 45 mm. embryo was sectioned serially and trans- 

 versely, but the changes showed so plainly that no reconstruction was 

 made of it. It is now impossible to trace the divisions between the 

 separate electric organs, there appearing one very large organ which 

 occupies the entire space between the eye and the brain and extends 

 from the dorsal surface to a point not far from the ventral surface 

 of the embryo, completely inclosing the eye-muscle (plate vii, fig. 3 

 and figs. 5-10). 



