216 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



increases in size, partly by^he adding on to it of new electroplaxes 

 formed at its periphery, but the marked growth which takes place 

 in the organ later on is due to actual growth of the individual units 

 which form it. Thus comparing a skate of 180 cm. length with one 

 of 45 cm. the individual electroplaxes are found to have increased in 

 size practically in the same proportion as has the body as a whole. 



The above description deals with the development of the electric 

 organ as it takes place in Raia batis. In other species of skate the 

 process appears to be similar as regards its main features, but it is 

 interesting to notice that the relative expansion of the front end of 



the muscle-fibre to form 

 the electroplax is much 

 less pronounced in cer- 

 tain species than is the 

 case in R. batis. 



Of the species so 

 far investigated R. radi- 

 ata shows the least ad- 

 vanced stage of evolu- 

 tion. In this species 

 (Fig. 119, A) the elec- 

 troplax is, as in vari- 

 ous other species (e.g. 

 R. circularis and R. 

 fullonica, Fig. 119), in 

 the form of a cup 

 rather than a disc. In 

 R. radiata the wall of 

 the cup is very thick 

 and retains throughout 

 life only slightly modi- 

 fied muscle structure. 

 The electric layer is 

 relatively feebly devel- 

 oped, the thick alveolar layer is represented by hardly modified 

 sarcolemma and the tail is only comparatively slightly degenerate. 



The skate has been taken as the basis for the description of the 

 development of the electric organ since the phenomena concerned 

 have been particularly clearly worked out in this fish. In the 

 Torpedoes the electric organ develops from muscles in the region of 

 the visceral arches by very similar stages. As regards the electric 

 organs of Teleosts our knowledge is still very insufficient. ' In 

 Mormyrids and in Gymnotus they are clearly modified portions of 

 the lateral muscles as in the skate ; in Astroscojms (Dahlgren) they are 

 believed to be derived from eye • muscles ; while in Malopterurus 

 though generally believed to be modified skin glands they are 

 believed by Dahlgren and Kepner (1908) to be more probably of 

 muscular origin. 



Fig. 119. — Illustrating the adult condition of the electro- 

 plax in (A) Raia radiata, (B) R. circularis, (C) R. 

 fullonica, and (D) R. batis. (After Ewart, 1892.) 



