The Origin of the Electric Organs in Astroscopus Guttatn*. 161 



and the current is more effective if they be long and narrow, since then 

 the resistance to the current is less. In Astroscopus the electroplaxes 

 are formed from four different muscles, running in four different direc- 

 tions. The necessity for orientation is therefore evident. The process 

 takes place between the 20 mm. and the 45 mm. stages. In the 20 mm. 

 embryo the electric tissue is still a part of the muscles, though sepa- 

 rating from them at the ends. In the 33 mm. embryo the organs are 

 quite separate from the eye-muscles, but still exist as four separate 

 organs, each organ lying in the position in which it came off from the 

 muscle, so that they lie in four different directions. The electroplaxes 

 of each separate organ lie approximately equal to each other in the 

 middle, but on the edges they may be found in all stages of orientation, 

 some of them having assumed the shape of horse-shoes, crescents, etc. 



From the first the electroplaxes are very evenly spaced in the lon- 

 gitudinal direction and closely interlocked in the other. The cells 

 elongate as they turn and the two processes occurring at the same time 

 probably account for the curious overlapping of the electroplaxes 

 described above. There appears to be a much larger number of the 

 short electroplaxes in the 33 mm. stage than there are of the longer 

 electroplaxes of the 45 mm. stage- doubtless another expression of this 

 peculiar turning which enables the microtome knife to cut them in 

 various planes, so that there appears to be a larger number of them than 

 there really is. This fact also lends grounds for the idea that the 

 electroplaxes in dropping from one row to another may cut through 

 an otherwise single electroplax, making two where there was but one. 



The process of orientation has been practically completed by the 45 

 mm. stage when the electroplaxes are found in their final position, 

 parallel to each other and to the body-surface. At the extreme edges 

 the electroplaxes never do completely straighten out, for it would seem 

 that the body is not wide enough to accommodate the rapid growth of 

 the electroplaxes, which continue to grow as long as the growth of the 

 fish continues. Another place where the electroplaxes do not become 

 strictly parallel is where the muscles pass through the electric organ. 

 There is always a considerable amount of space around the muscle and 

 the electroplaxes bend in toward each other at the edges, making a wall 

 around them. At the extreme dorsal and ventral surfaces of the organ, 

 also, the electroplaxes are crowded up closer than usual. With these 

 few exceptions, by the time the fish is 45 mm. long the arrangement of 

 the electroplaxes is strictly parallel and at even distances from one 

 another. 



The extraordinary regularity of the spacing is a very striking point. 

 The width of the area between the electroplaxes does not vary more 

 than 0.5 micron anywhere in the organ. Looking to the other electric 

 fishes for comparison, we find that, with the exception of Malopterurus, 

 all exhibit this same remarkable regularity, but that in some forms the 



