RECURRENT BRANCHES OF ABDUCENS NERVE 383 



gated cells are found. In the 9.6-mm. and 7.5-mm. embryos 

 of about the same age, the eye-muscle mass is not so clearly 

 defined, but the somitic muscle is continuous up to a point just 

 caudal to the vagus ganglion. In a sheep embryo of 7.2 mm. 

 (H. E. C, no. 1226), on the other hand, the oculomotor nerve 

 is large and can be traced to a well-defined eye-muscle mass, 

 while the somitic muscle ends some distance caudal to the vagus; 

 the abducens is just beginning, and turns almost immediately 

 forward. The same is true in rabbit embryos of 5 mm. (twelve 

 days), as regards the oculomotor and abducens nerves and the 

 disposition and condition of the muscle masses. A difference 

 exists, however, between sheep and rabbit in the extent of the 

 hypoglossal roots, which are confined to a caudal position in 

 sheep, but reach far forward in rabbit, in these early stages, 

 leaving only a small gap between them and the abducens roots. 

 The corollary of this appears in the frequent occurrence in the 

 latter embryos of anterior hypoglossal roots passing in front of 

 the vagus nerve. In the rabbit the earliest roots of the abducens 

 are almost pre-otic and actually nearer to the eye-muscle mass 

 than to the somitic muscle. In the pig the muscles have about 

 the same disposition as in man, but the oculomotor nerve arises 

 first and the eye-muscle mass may be considered to predominate 

 and attract the anterior abducens roots, which arise next in 

 order; the later, more posterior abducens roots, however, point 

 frequently toward the occipital muscles, to which they are 

 nearest, giving the small short-lived recurrent branches, which 

 are always separate roots, not interlaced with those of the 

 abducens proper. 



In the chick the order of emergence of the abducens rootlets 

 seems to progress usually from behind forward, as Belogolowy 

 said, and they are at first a continuation of the anterior hypo- 

 glossal roots. This would lead one to expect recurrent branches, 

 as the order is similar to that found in man, as opposed to sheep 

 and rabbit. The oculomotor nerve is, however, well developed 

 before the abducens appears, probably in conjunction with the 

 large and precocious eye, which is accompanied by precocious 

 eye muscles. The external rectus is already recognizable as a 



