1 90 Papers from the Marine Biological Laboratory at Tortugas. 



Further homology is seen in the disposition of the probably superfluous 

 myofibrils. In both forms they are relegated to a middle position in the 

 electroplax, while the apparently more important cytoplasm forms layers 

 on the anterior and posterior surfaces of the structure. Also, in both, the 

 now useless myofibril bundles are packed out of the way at right angles to 

 the axis of the electroplax, which remains the same as the former axis of 

 the muscle-cells that were used to form it. 



The only difference lies in the fact that the striation of the fibrils is 

 retained in the Mormyrus forms, while it is lost in Gymnarchus, the dark- 

 staining anisotropic substance apparently dissolving away. 



From what little can be predicted concerning the possible origin of the 

 electric tissue in the other teleost forms, it is probable that the Mormyridae 

 (including Gymnarchus) are the only fish in which the electroplax is formed 

 as a syncytium from more than one cell. In Astroscopus, Electrophorus 

 (formerly Gymnotus), and Malopterurus the structures show every evidence 

 of having been developed from single myoblasts with the exception of 

 Malopterurus, where it is a question as to whether they are not evolved from 

 gland-cells instead. 



SUMMARY. 



In conclusion it may be stated that — 



(i) The electric tissues of Gymnarchus are developed by the differ- 

 entiation of certain portions of its normal, striated muscle-tissues during 

 an embryonic or larval period extending from the ninth day to the forty- 

 second day of embryonic life. The critical period of this change takes place 

 in the neighborhood of the eleventh, twelfth, to fifteenth days. 



(2) The muscle fibers which go to form the electric tissue give up their 

 usual segmentation into myotomes, and first form eight long and con- 

 tinuous spindles which afterward segment, each into a lesser number of 

 masses, the electroplaxes. 



(3) The myofibrils lose their transverse striation and form a large inner 

 core for each electroplax. They lie in a wavy mass, mostly at right angles 

 to their former course. The active cytoplasm forms an outer layer which is 

 denser and stains deeper on the posterior than on the anterior surfaces. It 

 contains granules. 



(4) Each electroplax is made up of several muscle cells, 12 to 20 or more. 

 This is different from the two elasmobranch fishes, in which each muscle 

 cell forms only one electroplax. 



(5) The nerve is distributed on the posterior surface and ends in blunt 

 knobs that lie in cavities formed by the invagination of the surface of the 

 electric layer. The current probably runs from tail toward head, as in 

 Mormyrus. 



(6) The development of the tissue gives a strong clew to the probable 

 development of the electric tissues in the other mormyrid fishes. 



