384 JOHN LEWIS BREMER 



separate muscle in chicks of little over forty-eight hours' incuba- 

 tion, and in an embryo of about three days, 6.2 mm. (H. E. C, 

 no. 511), is well developed and with clear-cut edges, though the 

 abducens fibers have not yet emerged from the brain. The 

 large eye has also altered the position of this muscle, the caudal 

 end of which lies far caudal to the anterior edge of the trigeminal 

 ganglion, much nearer the otocyst than in mammals. The 

 occipital muscles in this embryo are also readily recognizable, 

 and end with a sharp edge medial to the vagus ganglion. The 

 abducens roots, as they emerge, would perhaps feel the attraction 

 of both muscles, the anterior group of them turning forward, the 

 posterior group backward, and the intermediate group indifferent, 

 as described by Belogolowy. These groups, however, include 

 all the rootlets' back to the hypoglossal nerve, whereas the 

 anterior alone remain permanently as the abducens nerve, and 

 since these have been influenced by the proximity of the external 

 rectus muscle, no recurrent branches in a strict sense are found. 

 Belogolowy figures fibers continuing the abducens caudally, to 

 be sure, but a study of their course and shape shows that they 

 are the remains of more caudal rootlets which were attracted 

 forward to join the abducens by loops, and which now are degen- 

 erating, by loss of their connection with the brain, as in the case 

 of the degenerating hypoglossal roots already referred to. In 

 other words, they are the remains of cranially directed fibers 

 from caudal rootlets not caudally directed recurrent fibers. 



The disposition of nerve roots and muscle masses in Lacerta 

 and Chrysemys is similar to that just described in chicks. The 

 permanent, cranial abducens rootlets are much nearer to the 

 large, external rectus muscle than to even the most cranial of 

 the occipital muscles, and the caudally disposed abducens root- 

 lets degenerate so rapidly that no recurrent branch becomes 

 established. 



In Acanthias new elements seem to influence the course of the 

 abducens fibers. The lack of a neck bend would remove the 

 occipital muscles relatively farther caudally, but the more com- 

 plete development of the postotic head cavities would probably 

 more than overbalance this; and the position of the nerve roots 



