1 10 HISTOLOGY 



These end-plates appear, in a section, as a rather closely set row of tiny, 

 transparent areas lying on the anterior surface of the electroplax (Fig. 

 105, el.n.e.}. The finer non-medullated ramifications of the nerve are 

 closely associated with the connective-tissue cells whose nuclei are to be 

 seen lying alongside of the branches of the fiber at many points near the 

 electroplax (Fig. 105, el.c.t.}. 



The shape of the electroplax is that of a rather thick disk with its 

 circular edge slightly thinned and bent upward (anteriorly). It has, 

 therefore, been called saucer-shaped or, where in other skates the bent 

 edge is higher, cup-shaped. Its shape varies much in the different spe- 

 cies of skates. 



The electroplax is composed of three layers, the two outer of which are 

 continuous around the edge and must be regarded as two (an anterior 

 and a posterior) specialized areas of a general outer layer. The middle 

 or inner layer is of a very different structure and much the thickest. It 

 forms a core or inner portion and is striated. These striations represent 

 cross sections of a series of undulating and parallel lamellae. They 

 correspond to similar structures in striated muscle, and under the highest 

 powers are seen to be formed of sheets of upright rods. 



With different fixatives and in different specimens the appearance of 

 these rods varies, much as does that of the muscle-rods. At times a 

 row of dots forms an equidistant line between them and again the rods 

 themselves appear double or very short, with two dots on either end. A 

 fine transparent fibril can be seen running at right angles to the lamellje 

 and connecting the rods into fibrillar structures like those of muscle. 

 This fibril probably is homologous with that of striated muscle tissue, and 

 the striation is due to the exact and equal arrangement of the anisotropic 

 and isotropic substances on the parallel fibrils. Their very small size 

 prevents satisfactory studies of their structure. 



The lamellae are very much denser and finer in Raja l&vis, and it is, 

 in this latter species, almost impossible to see the rods that compose an 

 anisotropic line. Here, too, there is far greater density and continuity 

 of the striation, and they are not so irregularly arranged as in ocellata. 

 The whole syncytium is surrounded by a delicate cell-membrane, the 

 electrolemma, which corresponds to the sarcolemma in a voluntary muscle 

 cell. 



While the anterior surface is flat the posterior surface is evaginated 

 into many papillae that vary in the individual electroplaxes as to width 

 and length. The posterior layer covers these papillae and follows all 

 their turns and bends. Where the width of a papilla is less than that 

 of a double thickness of the posterior layer, no striated or middle layer 

 exists except certain broken off and irregular portions of striated material 

 lying in the papillae. In Raja lavis the striated substance is pushed 



