The Origin of the Electric Organs in Astroscopus Guttatus. 167 



2. The four organs begin to differentiate before the embryo is 14 mm. 

 in length. The first indication of the change is a slight darkening of 

 the muscle-cells on one edge of the muscle, followed by a multiplication 

 of nuclei and the rapid growth of the cells. No such areas form on 

 muscles which do not form electric organs. 



3. The electroblasts are completely differentiated by the 20 mm. stage 

 and the organs have separated from the muscle by the 35 mm. stage. 



4. Between the 33 mm. and the 45 mm. stages the four organs, now 

 separated from the eye-muscle, orient themselves until their constitu- 

 ent electroplaxes become parallel to the dorsal and ventral surfaces of 

 the body of the fish and unite to form one large organ with a diameter 

 equal to one-third the diameter of the head of the fish. 



5. Each electroplax is formed by the modification of a single myo- 

 blast cell. 



6. The nuclei multiply and differentiate to form electric and motor 

 nuclei. The electric nuclei are wide and flat, and arranged in a regular 

 series on the negative surface of the electroplax. They have a definite 

 nucleolus and faint chromatic threads. The nutritive nuclei are 

 large and round, with several aggregates of chromatin but no definite 

 nucleolus. 



7. The cytoplasm differentiates into electric and nutritive layers. 



S. There are no easily visible myofibrillations, but the longitudinal 

 striations persist in the nutritive layer and are broken up into short, 

 curved striations, very definite in outline. They can be traced into 

 the electric layer. 



9. The vacuoles of the dorsal surface disappear, but those of the 

 ventral surface break through the membrane to form the papilla? and 

 the protoplasmic bridges which characterize the nutritive surface. 



10. The electroplax is 35 microns in width, one-third of which is elec- 

 tric and two-thirds nutritive material. It is twice as wide as the 

 electroplax of Torpedo, but only half as wide as that of the weak 

 electric fishes. 



11. The electroplaxes are arranged in a parallel series, but some are 

 carried down one or more layers in the process of orientation, so that 

 they form a part of the lower layers. 



12. The electric organ is innervated by that branch of the oculomotor 

 nerve which supplies the rectus superior muscle. It begins to differ- 

 entiate during the 20 mm. stage and is complete by the 33 mm. stage. 



13. The electric and third nerves leave the brain together from the 

 same nucleus, but separate before passing through the cranial wall. 

 The electric nerve passes bodily into the rectus superior muscle, the 

 fibers of which are torn apart by the process. 



14. The branches of the nerve reach every corner of the electric 

 organ and end on the dorsal surface of each electroplax. 



