Origiyi of Electric Tissues of Gymnarchns Niloticus. 165 



as is the case with its relatives, the various mormyrids. The natives of 

 Africa are much afraid of the creature, especially at nesting time, and one 

 of its Arabian names, "Abu rhad" meaning "father of thunders," might 

 seem to indicate perceptible electric powers. 



The embryos and young fishes put at my disposal by Dr. Kerr and Dr. 

 Assheton were five in number, and of these three were the suitable stages 

 from which this paper was worked out. The significant development of 

 the electric tissues in Gymnarchus takes place between the ninth day of 

 embryonic life, at which time the embryo possesses a fully formed and 

 complete musculature in the tail with no sign of an electric organ, and the 

 fortieth day of development, at which time it can be seen that the embryo 

 has developed its electric organs, out of a certain part of the previous muscu- 

 lature in the tail, to a degree that shows the farthest advanced electro- 

 plaxes in a practically adult condition. 



The most interesting and critical stages in this metamorphosis of muscle 

 into electroplax appear to take place within much closer limits, and stages 

 from the eleventh to the fifteenth day would include them. These signifi- 

 cant changes have been studied and drawn principally from an embryo 

 12 days of age, fixed in sublimate-acetic, and showing the changes very 

 much to my satisfaction. A point of interest and importance in this study 

 is that, in earlier embryos, the myotomes and electric spindles are youngest, 

 least developed, and growing fastest in the posterior part of the body or 

 nearest the tip of the tail; while in older embryos and in the adult the 

 greatest, most complete, and most characteristic development of the 

 electroplaxes is to be found in the end of the tail or at the posterior end of 

 the spindle. Thus the adult structures in the anterior part of the spindles 

 represent a somewhat inferior and less complete change of the muscle tissue 

 into electric tissue than the posterior parts of the same organs do. 



The same importance attaches to the fact that the rates of development 

 of the several spindles seem to vary. The lower median spindle starts 

 first to differentiate, extends farthest forward in the body, is larger than the 

 others when developed, and during early development is always in advance 

 of the corresponding parts of the other spindles. The upper median spindle 

 closely follows the lower. The ventral spindle is much behind the two 

 median ones, while the dorsal spindle represents the latest and weakest 

 development and is shorter than any of the others. These facts have made 

 it possible in the present study to get many stages of development from 

 very few embryos. 



STUDIES OF AN EMBRYO NINE DAYS OF AGE. 



This little fish was 26 mm. in length and, while the egg-membrane had 

 been ruptured and cast away, the animal was still forced to remain in its 

 nest because of its huge, elongate yolk-sac, still unabsorbed, and because 

 of its otherwise undeveloped organs of alimentation, locomotion, etc. 

 The posterior part of the body was carefully cut into four portions (see 



