STUDY OF AN EMBRYO 9.5 MILLIMETERS IN LENGTH.
The head of this embryo was sectioned serially and transversely and reconstructed with 2 mm. wax at a magnification of 500 (plate r). Some of the eye-muscles of this little embryo are already beginning to show a change, although no definite electric tissue has yet formed. The rectus externus, rectus internus, and rectus superior muscles show a definite darkened area on the side away from the eye-ball, which, upon examination, proves to be composed of cells smaller than the normal muscle-cells. Both the nuclei and the cytoplasm of these cells take the stain much more densely than the other cells, but this is the only sign of any modification of the cell structure (plate v, fig. 2).
Sections of a 4 mm. embryo were also studied with regard to this particular point, although the specimen was too poorly preserved for any more careful investigation. In this embryo areas corresponding to those mentioned above were found on the same muscles and with the same topographical relation to the muscle-cells. They had the same dense stain, but were very much smaller in area, as indicated in figure 1 of plate v.
A few embryos of the species Batrachus tau were then sectioned in order to observe the muscle-tissues, as it was thought possible that the smaller and more numerous cells might represent the generating area of the muscle itself and have no connection with the electric tissue. In these embryos such areas were found on several of the muscles, but a very interesting point lay in the fact that in every case this area of generating tissue lay on the opposite side of the muscle from that on which the electric organs of Astroscopus are derived, i. e., on the side toward the eye-ball. In Astroscopus, however, this area corresponds in every case with the position from which the electric organ is to be generated; moreover, no such areas could be found in Astroscopus on the muscles not forming electric tissue. For this reason I feel certain that the darkened areas represent the first beginnings of electric tissue. The possibility also arises that in the darkened areas on the muscles of the young Batrachus we may have a step in the evolution of the electric organs in the toad-fish group.
The innervation of the eye-muscle in the 9.5 mm. embryo is perfectly normal and no nerve-endings are seen even to approach the electric tissue. A résumé of the normal eye-muscles, their origin, insertion and innervation will here be given for reference (Herrick 61 and Wieders heim 94). The eye-muscles are six in number—rectus internus, exter nus, inferior, and superior, and obliquus inferior and superior.
The rectus internus, externus, and superior originate in the subcra nial canal at the extreme caudal end of the basioccipital. The rectus externus originates in the extreme caudal end of the canal and runs dorso-laterad to its insertion on the base of the eye-ball. It receives the entire abducent or sixth nerve.
The rectus internus originates near the caudal end of the canal in a groove on the dorsal side of the parasphenoid bone and runs along the inner sides of the orbits to its insertion on the cephalic border of the eye-ball. It receives a branch of the oculomotor nerve, or the third nerve.
The rectus superior originates in the cephalic end of the subcranial canal from the parasphenoid, under the rectus internus, and from the membranous roof of the canal over the same. It runs close to the rectus internus to the end of the canal, turns dorsal and laterad, and crosses the rectus inferior and optic nerve to reach its insertion. It receives a branch of the oculomotor nerve.
The rectus inferior arises from the basisphenoid bone and runs over the rectus internus, rectus superior, and optic nerve to its insertion. It receives a branch of the oculomotor.
The obliquus superior and inferior muscles arise far cephalad from the internasal cartilage and run caudal to their insertions.
The obliquus superior arises farther cephalad than the interior muscle and is inserted on the dorsal surface of the eye-ball. The obliquus inferior originates a little farther back from the dorsal wall of the car tilage and lies nearer the median line. It passes mediad from the superior muscle direct to the ventral side of the eye. The obliquus superior is innervated by the entire trochlear or fourth nerve and the obliquus inferior receives a branch of the oculomotor.
The sixth or abducent nerve arises from a center in the medulla some distance from the median line near the lateral surface of the brain. It passes through a foramen in the cranial wall into the dorso-lateral angle of the subcranial canal and passes direct to the lateral face of the rectus externus, which it enters.
The trochlear or fourth nerve arises from a separate center in the hind-brain just caudal to that of the third nerve. It leaves the brain behind the optic lobes and continues intracranially a short distance, when it pierces the cranial wall, turns dorsad, and moves direct to the dorsal side of the obliquus superior.
The oculomotor or third nerve is a large nerve arising in the mid brain from the nucleus near the median line in part menial of the large fasciculus longitudinalis dorsalis. There is no trace of the differen tiation of the large electric motor-cells at this stage. The nerve leaves the brain and gives off its first branch to the obliquus inferior. Both branches pass at once through the cranial wall just menial of the rectus externus. The branch to the obliquus inferior turns ventrad and passes the rectus externus, internus, and inferior on its way to the ventral face of the obliquus inferior. The main branch gives off another branch which divides and goes to the rectus internus and inferior, but the main branch itself divides again, both branches entering the rectus superior muscle, a point of significance in light of further investiga tions.