The electric organ develops in four parts on each side of the eye, each of which is called an electric organ for convenience during its develop ment. The rectus externus is the largest of the six eye-muscles and has a proportionately large electric organ. It is the first of the four to begin development. The central part of the tissue forms a broad band lying close to the muscle and curving from its dorsal to its ven tral surface in a spiral. The ends of the tissue separate from the mus cle ventrally, just caudad to the origin of the muscle, running caudad and ventrad, and dorsally, near the point of insertion, running craniad. This separation of the organs from the muscle with the turning and bending of the organs is accomplished by the elongation of the electro blasts at the point of separation. The growth of the cells, there fore, begins at either end of the organ and later progresses toward the center. Also, the electric organ of the rectus externus muscle, which was the first to begin development, is the first to begin to separate from the muscle-tissue (plates n and in, R. ex.).
The long, narrow electric organ of the rectus internus lies close to its surface and curves spirally from a point cranial and medial around the dorsal to the ventral side of the muscle. Dorsally the organ re mains closely attached to the muscle, but ventrally it separates to run caudad a short distance. It is the second of the four electric organs to begin development and the second to begin to separate from the muscle (plates n and in, R. int.).
The electric organ of the rectus superior muscle lies close to the median side and does not curve around it nor separate from it. The obliquus superior is the last of the four muscles to develop an electric organ, which is correspondingly small, forming a short, broad band, caudad and close to the muscle, about halfway between its origin and insertion (plates n and in, R. su., O. su.). In the figures the electric area is stippled.
The number of electroblasts do not increase after once being formed. Each electroblast is the product of a single cell and each electroplax is the product of a single electroblast, so that the amount of electric tissue to be contributed by the various muscles has already been determined. It was most unfortunate that no embryo could be obtained represent ing a stage between that of the 20 mm. embryo and of the 33 mm. em bryo, as it is between these two stages that the actual separation of the organs from the muscles takes place. The position of the electric organs in the 20 mm. stage, however, goes a long way toward explain ing the real process, so that we are safe in drawing pretty definite con clusions without the intermediate stage.
Except that the nerves branch much more profusely in all of the muscles, no change has been noted in any but that branch of the ocu lomotor nerve which goes to the rectus superior. As noted above, the main branch of the oculomotor goes to the rectus superior in two divi sions. The larger of these two divisions enters the muscle somewhat posterior to the smaller one, and is seen to have enlarged to nearly twice its original diameter. The small branch goes to all parts of the muscle, but the large one goes to the middle of the muscle, where it divides into many parts, all but one of which are seen to approach the electric organ of this muscle and stop. There is a considerable space between the muscle and the electric tissue, and the axones of the nerves must cross this space to enter the electric organ. Although the mate rial was not favorably fixed for the study of nerves, a few fibers could be traced across the intervening space and into the electric organ.
Unfortunately, the nerve-endings could not be seen in any of my material and I have had to pass over this point until more material can be obtained. The last division of this branch, which does not approach the electric organ, passes directly through the rectus superior muscle and can be traced ventrally and mesad until it approaches the electric organ of the rectus intemus muscle. It was impossible to trace any of the fibers of this branch into the electric organ, but the nerve itself could be traced until it touched the electric tissue, so that there can be no doubt that the axones do pass in and that with better material they could be seen to be distributed among the electroplaxes of this organ as well as that of the rectus superior. This is the first indication that the nerve which innervates the rectus superior muscle is to innervate the electric organs also, but it is not yet clear that it is the only nerve which will do so, although it is clear that the electric organ of the rectus superior is not the only one to be innervated by it. Later it sends branches to all four of the electric organs.
A few electric motor-cells are found in the motor nucleus of the third nerve. It is of interest in this connection that the rectus superior muscle is neither the first of the muscles to develop electric tissue nor does it contribute the largest amount of tissue to the organs. On the other hand, it does not contribute the smallest amount either, so there is no appar ent reason why the nerve of this particular muscle should be called on to do all the work. It might have been expected that the sixth nerve, which innervates the rectus extemus muscle, would contribute to the electric organs, since that muscle has the largest of the four organs, or (on the other hand) that the trochlear or fourth nerve to the obliquus superior muscle would contribute, since that muscle has the smallest amount of electric tissue. As a matter of fact, subsequent study has shown that there are no electric motor-cells to be found anywhere in the brain except in the nucleus of the third nerve, no fibers from any other nerve or from any other branch of that nerve even approaching the electric tissue. When the third nerve emerges from the cranial wall the first of the muscles which it is to innervate which crosses its path is the rectus superior; it is also the first muscle which it enters. Whether this fact has any significance for the fact that this particular branch forms the electric nerve or not, I can not say, but no other explanation presents itself.
The circulation in the head of the 20 mm. embryo appears to be per fectly normal, presenting no points of difference from that of other non-electric toad-fishes. The external carotid artery runs craniad just dorsal to the rectus inferior muscle, and sends off small branches to the various eye-muscles on the way to the eye-ball; it then runs between the two oblique muscles and craniad below the eye-ball. The vessels to the eye-muscles send no branches to the electric organs, either before or after entering the muscles. The ophthalmic artery branches from the pseudobranch to run craniodorsad to the eye-ball, which it enters with one of the ciliary nerves from the fifth cranial. It gives off a small branch just dorsal to the obliquus inferior muscle, which anastomoses with the corresponding artery on the other side, but this branch has no connection with the electric organs.