KLKOTKICAL OKGANS 



213 



entirely, while UK; production of electrical disturbance has become 

 predominant, We have here an excellent example of the principle 

 of "substitution of functions," which is constantly at work during 

 evolution, the previously predominant functions of organs becoming 

 subsidiary <>i % tailing in ID abeyance and being replaced by functions 

 which were previously subsidiary. 



The development of the electrical organ can be conveniently 

 studied in the case of the Skate (Raid) of which the most complete 

 description has been given by Ewart (1888, 1889, 1892). In this 

 animal the electric organ forms an irregularly spindle-shaped body 

 which lies embedded in the lateral muscles on each side of the tail 

 region. It varies in size in different species and is distinguishable 

 to the naked eye from the muscle by its more gelatinous appearance. 



FIG. 117. Side view of skull and niyotonu-s of Lepiifoxire/t, stage 38. (From Agar, 1907.) 



Cartilage dotted, myotomes indicated by outlines, nerves black, a.o.p, antorbital process ; "'/,/. 

 caps, auditory capsule ; br.n, brachial nerve ; c.ph.n, nerve to dorsal portion of constrictor of pharynx ; 

 car. hi/, coracoliyoid muscles; hy, hyoid arch; hi/po<j.u, hypoglossal nerve; M.I/, M.:, MA, .Vf.2, etc., 

 myotomes; mnml, Mcckrl's cart ilage ; im*. my/.--, nasal capsule; urc. /<//, occipital arch; occ. rib, 

 occipital ril; //, pectoral girdle; ^ntnl, quadrate; //, r, nerves; 1, 2, 3, etc., spinal nerves; 3 67-, 

 branch from 3 to brachial nerve. 



On examining transverse sections through the tail it is seen that 

 the electric organ occupies the place of the middle one of five super- 

 imposed portions into which the muscle is divided. And this clearly 

 suggests, as Babuchin first pointed out, that the electric organ is 

 morphologically part of the muscular system. That this is actually 

 so is placed beyond dispute by the facts of development. In an 

 embryo of JR. batis about 7 cm. in length the position of the future 

 electric organ is indicated by a slight modification of the muscle 

 fibres, inasmuch as some of these (Fig. 118, B) show a tendency to 

 assume the shape of a club, the anterior end of the fibre being 

 slightly thickened. In contact with this thickened end is a mass of 

 protoplasm crowded with nuclei. This represents the motor end- 

 plate which has assumed a terminal position. 



In a slightly older embryo (Fig. 118, C) the club-shaped fibre of 

 the preceding stage has become further modified, the anterior end 



