EXPLANATIONS OF THE PLATES. 

 Plate i. 



Fig. I. Transverse section through body of 9-day-old embryo of Gymnarchus in region C. 

 L.M, position of the large muscle cells that will eventually change into the 

 lower median electric spindle; V.F, location of large vertebral fibers; XX XX, 

 locations of electric spindles in an adult fish with reference to bony structures, 

 blood-vessels, and spinal cord; N, canalis centralis; Bi and B2, caudal 

 blood-vessels. X no. 



Fig. 2. Muscle-tissue in transverse section from 12-day-old embryo of Gymnarchus to 

 show typical muscle cells. A, one of oldest fibers with two strong fibril 

 bundles; B, growing muscle cell with fibrils being laid down in a ring; C, 

 extreme upper edge of myotome with youngest cells at top. X 1150. 



Fig. 3. Transverse section through group of larger muscle cells found at L.M. in fig. I. 

 Under greater magnification. For explanations see text. X 1150. 



Fig. 4. Longitudinal vertical section of C region in 9-day-old embryo, ventral portion. 

 Between marks O O are seen same parts of seven myotomes as are shown 

 at L.M. in preceding figure in transverse section. Note heavy development 

 of these cells. A line drawn between marks © © would indicate location of 

 seven groups of vertebral fibers in next few mediad sections. 



Plate 2. 



Fig. 5. Transverse section of body of 12-day-old Gymnarchus embryo in region CCu D, 

 dorsal electric spindle; U.AI, upper median spindle; L.M, lower median 

 spindle; V, ventral spindle; Conn.T, connective-tissue septum; N, canalis 

 centralis. Bi and Bi, caudal blood-vessels. X no. 



Fig. 6. Three degenerating muscle cells from another (3) zone in fig. 12. X 1440. 



Fig. 7. Electric spindle showing first stage where electric muscle cells can be called an 

 electroplax. Complete coalesence has taken place. Nuclei are in peripheral 

 layer. Myofibrils are concentrated in middle to form central fibrillar core of 

 structure. X 1150. 



Fig. 8. Transverse section of upper dorsal part of spinal cord in a Gymnarchus, embryo 

 9 days old. Neuroblasts, in region that will later show electric motor-cells, 

 show some little differentiation and growth. Near, and above, this region is 

 one of the transitory giant ganglion cells. X 940. 



Plate 3. 



Fig. 9. Transverse section of left ventral spindle from region C in embryo of 12 days. To 

 left of dotted line are normal muscle cells, to right are degenerating muscle 

 cells, in the midst of which, and surrounded by dotted circle, are a group of 

 muscle cells going through the first steps of transformation into electric 

 tissue. X 1 150. 



Fig. 10. Transverse section of ventral electric spindle in process of formation in region B, 

 of 12-day-old embryo of Gymnarchus. At left of figure two muscle cells 

 begin to degenerate, while just to right of upper one of this pair a muscle cell 

 is seen in advanced stage of atrophy. Connective-tissue capsule beginning 

 to form. Electric muscle cells rather more coalesced than in fig. 9. X 1150. 



Fig. II. Transverse section of upper middle electric spindle in same section as fig. 10. 

 Rapidly degenerating muscle cells to left. Further coalescence of electric 

 muscle cells than in fig. 10. Myofibrils weakening but still in separate 

 groups. X 1 150. 



Plate 4. 



Fig. 12. Longitudinal vertical section of dorsal part of region D of 12-day-old embryo of 

 Gymnarchus, showing relations of muscle tissue and electric tissue. Bracket 

 I indicates zone of degenerating muscle fibers on inner m.edian edge of five 

 myotomes, on ventral side of median connective tissue; zone 3 shows atrophy- 

 ing muscle fibers on inner median edge of myotomes, dorsad of median con- 

 nective tissue; zone 4, rudiment of upper median electric spindle; zone 5, 

 same of dorsal electric spindle; zone 6, muscle tissue from ventral spindle to 

 dorsal outer edge; zone 7, connective tissue; zone 8, dorsal epithelium. 

 X 140. 



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