it ELECTEICAL OKGANS 215 



saucer shape with its concavity posterior and composed of numerous 

 closely packed lamellae. It forms what is termed in the fully de- 

 veloped -organ the striated layer. On its anterior face the striated 

 layer is covered by the end-plate, now known as the electric layer, 

 while its posterior face is covered by a thick layer of richly nucleated 

 protoplasm which, from the deeply pitted character of its posterior 

 surface, is known as the alveolar layer. From this passes back- 

 wards the main part of the muscle-fibre which shows symptoms of 

 degeneration especially in the portion next the alveolar layer where it 

 becomes vacuolated. Whether the alveolar layer represents, as 

 seems probable, a localized thickening of the sarcolemma is not 

 clear from the descriptions. 



In the fully developed condition (Fig. 118, E) the muscle-fibre 

 has become converted into the functional electroplax (Dahlgren, 

 1908) or electrical unit. What was the head of the mace in earlier 

 stages is now expanded to form a broad thin circular disc, lying 

 perpendicular to the long axis of the body — the stem of the mace 

 having degenerated into an apparently insignificant and functionless 

 vestige (Fig. 118, E) or having disappeared entirely. The electro- 

 plax is formed of the striated layer which is almost flat except 

 round its edge where it is bent in a tailward direction. It is com- 

 pletely ensheatbed in syncytial protoplasm, that on its posterior 

 face forming the alveolar layer, probably nutritive in function, that 

 on its anterior face forming the electric layer. Into the latter there 

 pass the numerous end -twigs of the nerve-fibres, the superficial 

 (i.e. headward) layer showing a characteristic fibrillation of the 

 protoplasm in a direction perpendicular to the surface (nervous 

 layer — Ewart) in contrast to the deeper portion in which the proto- 

 plasm is granular and nucleated (nuclear layer — Ewart). The tail- 

 like vestige of the posterior portion of the muscle-fibre is directly 

 continuous with the striated layer. With the latter it represents 

 the contractile portion of the original muscle-fibre, while the 

 ensheathing protoplasm whether electric layer, or alveolar layer, or 

 sheath of the tail-like vestige, is probably to be regarded as repre- 

 senting the superficial portion of the sarcoplasm. 



As the muscle-fibres pass through the above-described modifica- 

 tions, the connective tissue between them increases in quantity and 

 becomes condensed between the electroplaxes in such a way that 

 each electroplax becomes enclosed in a disc-shaped compartment. 

 The walls of this fit close to the electroplax round its edge while, on 

 the other hand, the anterior and posterior walls are separated, 

 especially the latter, by a wide space from the face of the electro- 

 plax. This space is occupied by connective tissue with sparsely 

 scattered cells and a jelly-like appearance. That on the anterior 

 side is traversed by the very numerous nerve-fibres which branching 

 dichotomously pass towards the electric layer, while that on the 

 posterior side is traversed by blood-vessels. 



During the earlier stages of development the electric organ 



