158 Papers from the Department of Marine Biology. 



three or four times as wide as either of the other nerves which supply 

 the eye-muscles. The third and electric nerve leave the brain together 

 from the same center, which is now filled with the large electric nuclei, 

 occupying much more space than the ordinary third nucleus (plate 

 vn, fig. 5) . They appear mesial, dorsal, and ventral of the fasciculus 

 longitudinalis dorsalis. The cells are large and egg-shaped, with very 

 granular cytoplasm. A large nucleus lies at the broad pole of the cell- 

 body. It is fairly round in shape and has a few strands of chromatin 

 running through it, as well as a very small aggregate of chromatin 

 which is fairly persistent. At the pole of this nucleus correspond- 

 ing with the broad pole of the cell is a very definite round nucleolus 

 with a vacuole on one side. This body takes the chromatin stain 

 heavily, while the rest of the nucleus takes it very lightly and the 

 cytoplasm of the cell with only medium density. In the adult the 

 cells measure 32 by 56 microns at their broadest point. 



The third nerve branches from the electric nerve immediately upon 

 leaving the brain, but continues to run close to its side until it has 

 passed through the cranial wall (plate vn, fig. 6). In speaking of 

 the electric nerve, it will be understood that that branch of the third 

 nerve which goes to the rectus superior is included and, for that mat- 

 ter, that the electric and third nerve are one from their origin, but are 

 given separate names for convenience. The other branch will be known 

 as the third nerve. After passing through the cranial wall they go 

 direct to the rectus superior muscle. The third nerve then passes 

 laterad around the muscle, dividing at the same time into three 

 branches for the three muscles which it is to innervate (plate vn, 

 fig. 4) . The small ciliary nerve to the fifth cranial has up to this time 

 been in such close connection with the third nerve as not to be distin- 

 guishable from it, but now separates and can be traced into the sclera 

 of the eye. The three branches of the third nerve pass directly through 

 the electric organs in their passage to the muscles, but otherwise their 

 behavior is perfectly normal and as described for the 20 mm. embryo. 



The electric nerve passes with its entire bulk into the rectus superior 

 muscle, the branch which is to innervate the muscle itself being sepa- 

 rated from it just previous to entering. The nerve is twice as wide as 

 the muscle itself, so that it tears the tissue of the muscle apart as it 

 enters, and when it divides in the muscle its branches separate the 

 fibers until they lose all connection with each other. One very small 

 branch of the electric nerve innervates the muscle ; the rest pass out 

 and go in all directions through the electric organs, dividing and redi- 

 viding as they go until the branches are no wider than ordinary nerves. 

 Some of the branches turn back dorsally, go around the organs, and 

 enter from the dorsal end; others pass directly through the organs 

 mesially and laterally and enter the organs from the ventral end, while 

 still others push their way between the organs in a ventral direction, 



