114 HISTOLOGY 



In an embryo of about 10 or u cm. in length, enough variety and ad- 

 vancement in the development of the electroplax can be found to supply 

 all needed steps. The muscle fiber is seen here with the anterior enlarge- 

 ment much wider and heavier and showing the form and structure of 

 the completed electroplax (Fig. 107, C). The posterior end, on the other 

 hand, is arrested in its growth and development and in the older cells is 

 actually shriveled into a ribbon-like form that still clings to the electro- 

 plax (Fig. 107, D). It retains its striated fibrils for a time, but they are 

 gradually absorbed until nearly lost. This muscle-cell remnant is 

 entirely missing in most other skates. 



The further following changes have also taken place in the somewhat 

 older stages. The motor nerve-ending of the young muscle fiber has 

 moved to the anterior end of the developing electroplax and has devel- 

 oped to form the electric nerve-ending which lies at this time in a saucer- 

 shaped depression on the end of the structure. The striated portion of 

 the muscle fiber has widened and shortened to form the striated layer of 

 the electroplax, meanwhile changing its comparatively wide and straight 

 bands of anisotropic substance to narrower and curved bands. These 

 striations of the electroplax are still, however, as strictly parallel as were 

 the muscle striations. The changes undergone by the striation have not, 

 as yet, been properly investigated. 



The electric layer has been formed from a layer of the muscle-nuclei 

 lying in the undifferentiated cytoplasm of the electroplax, and this electric 

 layer has extended over the edges of the electroplax to become the nu- 

 tritive layer on the posterior side. This nutritive layer has become evagi- 

 nated into a number of papillae of considerable length, extending into the 

 electric connective tissue and anastomosing with one another to a con- 

 siderable extent. 



So we see, in this development, the visible and undoubted steps of 

 the change of a muscle fiber into an electroplax ; thus abundantly cor- 

 roborating the surmises that this was the case in the electric tissues of 

 Mormyrus and the other forms, excepting only Malapterurus. 



The development or histogenesis of the electroplax in Torpedo can 

 again be compared with the process in Raja to great advantage. The 

 organ begins in this latter form as a set of vertical bundles of long thread- 

 like cells each with one nucleus (Fig. 108, A). These cells acquire a faint 

 striation as does a young muscle cell, and then the lower end begins to 

 enlarge, and the nucleus which is near the lower end begins to divide 

 (amitotically probably) into the large number of nuclei that are after- 

 ward found in the electroplax (Fig. 108, B). The club-shaped end 

 rapidly widens into the plate-like electroplax, while the upper end of the 

 fiber atrophies and is seen no more. The striation persists for but a 

 short time in its ventral edge and then disappears also. The striation 



