Origin of Electric Tissues of Gymiiarchtis Niloticus. 193 



Fig. 13. Longitudinal vertical section of ventral part of region C of 12-day-old embryo of 

 Gymnarchus. Zone i indicates ventral epithelium; zone 2, ventral outer 

 portions of five myotomes; zone 3, lower median electric spindle which is 

 dividing into three parts in five vertebral segments; zone 4, inner degener- 

 ating muscle fibers of five myotomes; zone 5, projected outline of vertebral 

 segments. X 140. 



Fig. 14. Motor nerve cells from spinal cord of 42-day-old larva of Gymnarchus. A, B, 

 and C, three electric motor nerve cells. D and E, two muscle motor nerve 

 cells. From region E. X 1500. 



Plate 5. 



Fig. 15. More highly magnified part of fig. 12. Brackets indicate same zones. X 760. 



Fig. 16. Highly magnified part of transverse section through an electroplax from C region 

 of 42-day-old larva of Gymnarchus. Curving bundles of modified myofibrils 

 are seen in core and cut at various angles. Outer or electric layer is fixed and 

 stained to show finer outer granules or electrochondria. Vacuoles as well as 

 larger meshes of the linom are visible on inner edge of electric layer. Elec- 

 tric nerves are cut in sections at many angles and show medullary or connec- 

 tive-tissue sheath and its nuclei. At two points branching of these nerve fibers 

 is visible and at four points its contact with electric layer of electroplax is 

 to be seen. X 1500. 



Plate 6. 



Fig. 17. Longitudinal section of a young electroplax from B region of 12-day-old embryo. 

 Segmentation of electric tissue is now complete into unit electroplaxes of 

 which figure is good example. Central fibril bundle beginning to lose its 

 transverse striation. Structure is as long as three muscle segments or bony 

 segments, as near-by spinal processes show. This represents oldest stage in 

 embryo of this age. X 210. 



Fig. 18. Longitudinal section of young electroplax from C region of 42-day-old larva of 

 Gyvmarchus. Fibrils have lost all striation and become waved in middle 

 part. Cytoplasm shows strong differentiation at two poles (ends). Anterior 

 end strongly vacuolized. Papillse begin to grow from ends of middle part 

 and nerve-endings have become established at junction of posterior third 

 with middle third. This shows youngest stage in 42-day-old larva and is a 

 successively later step in dift'erentiation than fig. 17. X 210. 



Fig. 19. Longitudinal section of an electroplax from E region of 42-day-old larva of Gym- 

 narchus. Fibral bundle much "waved" in middle third, which now begins 

 to show form of electroplax. Papillae more developed. Nucleus and some 

 cytoplasm inclosed in fibrillar core. X 210. 



Plate 7. 



Fig. 20. Longitudinal section of another electroplax from further caudad in E region of 

 same larva of Gymnarchus. This represents oldest stage accessible and is 

 regarded as almost adult in its histological characteristics. Two nuclei 

 shown in fibrillar core; one of these shows characteristic swelling. 



Plate 8. 



Fig. 21, Transverse section of body of 42-day-old embryo of Gymnarchus from D region. 

 Greater part of muscle tissue is here transformed into electric tissue. Section 

 passes through very last part of dorsal fin. Electroplaxes cut at different 

 levels and in one case section passes through right ventral spindle showing 

 only electric connective tissue. D, dorsal spindles; U.M, upper middle 

 spindles; L.M, lower middle spindles; V, ventral spindles; Bi and Bi, 

 caudal blood-vessels; N, neural canal. Ni, N2, N3, and Nt, electric nerves; 

 iVs, lateral line nerve. X no. 



Fig. 22. Part of transverse section from middle part of an electroplax in E region of sam.e 

 larva of Gymnarchus. This figure serves well to show larger granules that- 

 lie deeper in electric layer, also connective-tissue sheath or wall of electric 

 spindle. X 1500. 



Plate 9. 



Fig. 23. Longitudinal section from E region, showing parts or whole of 11 electroplaxes 

 lying in dorsal, upper middle, and lower middle spindles. Some remaining 

 muscle fibers are visible at sides. Figure serves to show that there is no 

 heavier, transverse connective-tissue wall lying in electric connective tissue, 

 as is found in Mormyrus. This emphasizes secondary segmentation found in 

 Gymnarchus. X no. 

 13 



