The Origin of the Electric Organs in Astroscopus Guttatus. 157 



can be seen, however, that the four organs are still distinct. Together 

 they occupy the entire space between the eyes and the brain, and 

 although they have been derived from only the dorsal muscles they 

 extend almost as far in the ventral direction as in the dorsal, the only 

 difference being that the ventral ends of the organs are narrower and 

 more pointed than the dorsal ends (plate vn, fig. 2). 



The electroplaxes have assumed an elongate shape, being in general 

 from three to four times as long as they are broad. They still have 

 quite a thick appearance, however, for while they have been growing 

 rapidly in one direction they have not become any narrower in the 

 other, the width of these electroplaxes being only slightly less than that 

 of the original electroblast. They are arranged in parallel rows, but 

 since all four of the organs slant in a different direction, the rows are not 

 parallel to the surface of the embryo, and on the whole they are only 

 fairly regularly arranged. Vent rally the surface is entirely cut up into 

 papilla? and many vacuoles are seen in the process of forming more 

 (plate vi, fig. 2). The dorsal area shows a decided change in structure. 

 The surface has become flat and smooth, the indefinite area of the 20 

 mm. embryo having completely disappeared. A few scattered vacu- 

 oles persist, but they are very small and round. The three layers of 

 the adult electroplax have now been formed : the ventral layer, which is 

 nutritive in function, with scattered nutritive nuclei, many papillae, 

 vacuoles, and perfectly straight striations; the middle or intermediate 

 layer, with no nuclei and no vacuoles, but with an abundance of stria- 

 tions; and the dorsal layer, with a few small, round vacuoles, a few 

 pretty definite striations on its ventral edge, and a very regular series 

 of oval electric nuclei. The rods described by Dahlgren and Hughes 

 have not yet appeared. 



In the previously described embryo some of the nuclei had rounded 

 up close to the membrane, while others remained free in the cytoplasm, 

 but no difference in the structure of the two kinds of nuclei could be 

 noted (plate vi, fig. 1). It is now evident that those nuclei which were 

 close to the edge of the membrane have become flattened to fit the 

 space, so that they appear quite oval in shape, while the nuclei which 

 remained in the cytoplasm have become the round nutritive nuclei. 

 They are less numerous than the electric nuclei and are not regularly 

 placed in a series, but scattered through the cytoplasm of the nutritive 

 layer. The electric nuclei have each a single nucleolus from which the 

 chromatin runs out in fine strands through the caryoplasm, whereas 

 the nutritive nuclei have several aggregates of chromatin but no well- 

 defined nucleolus. 



The electric nerve is now well developed from the branch of the 

 oculomotor which supplies the rectus superior muscle. It has assumed 

 such large proportions that the rest of the third nerve has a diameter 

 only one-fourth that of the electric nerve, and the third nerve itself is 



