156 Papers from the Department of Marine Biology. 



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 externus 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 

 ]Dath 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 

 A'arious 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 cihary 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. 



STUDY OF AN EMBRYO 33 MILLIMETERS IN LENGTH. 



The head of the 33 mm. embryo was sectioned serially and trans- 

 versely and reconstructed in 1 mm. wax at a magnification of 45. 

 Four well-developed electric organs appear, completely separated from 

 the eye-muscles (plate iv) . They are quite separate from each other 

 dorsally and ventrally, but in the center they are so closely folded 

 together that at times only a thread of connective tissue separates 

 them and without the previous studies it would be impossible to 

 determine from which muscle the different organs have been generated, 

 or in fact that they had been generated from muscle- tissue at all. It 



