114 Development of Occipital Xcrves in Iluniau Embryos 



roots of phvlogoneticall)' lost occipito-spinal nerves, which luive become 

 fused into a single trunk. 



Conclusions. 



1. The tenth and eleventh cranial nerves are parts of the same complex, 

 both possessing mixed motor and sensory rootlets, together with root 

 ganglia derived from the same ganglionic crest. 



2. During the progress of development of this vago-accessory complex 

 the cephalic end becomes predominantly sensory, and the caudal end 

 becomes predominantly motor and also more spread out. This produces 

 a difference in the appearance of the two portions which has resulted in 

 their being considered as two independent structures. The cephalic por- 

 tion forms the vagus or tenth cranial nerve, and the caudal portion the 

 n. accessorius Willisii or eleventh cranial nerve. The old nomenclature 

 is retained, and in so doing the term eleventh cranial nerve is used as 

 synonymous with n. accessorius vagi plus n. accessorius spinalis. 



3. The root ganglia of the tenth and eleventh cranial nerves do not 

 present a definite segmental arrangement. 



4. The trunk ganglia of the ninth and tenth cranial nerves (gang, 

 petrosum and gang, nodosum) when first identified are not definitely 

 connected with the root ganglia of the same nerves, and they differ from 

 the root ganglia in having an. arrangem.ent segmentally related to the 

 gill arches, and possessing rudimentarv sense organs. 



5. The ganglia found on the rootlets of the eleventh cranial nerve are 

 the counterpart of the root ganglion or jugular ganglion of the tenth. 

 They do not reach the high development of the latter, though traces of 

 them persist in the adult. They are to be distinguished from the pre- 

 cervical ganglion of Froriep, whicli represents an extra spinal ganglion. 



G. The eleventh cranial nerve extends caudalward into the spinal region 

 to the third or fourth cervical segment, in some cases further ; the extent 

 and variation in the embryo is the same as in the adult. The caudalward 

 invasion of this nerve is phylogenetic, and not ontogenetic. 



7. The hypoglossal nerve in young embryos closely resembles the 

 ventral roots of the adjacent cervical nerves, and is segmentally continu- 

 ous in the same line with them. That a phylogenetic retrogression has 

 removed the dorsal roots, which they seem to have at one time possessed, 

 is evidenced both by the occasional presence of a Froriep ganglion and by 

 cases in which the retrogression has gone still further caudalward, and 

 has removed the dorsal root of the first cervical nerve. 



8. The ramus descendens hypoglossi is developed in some cases before 

 the hypoglossus has received any connecting branches from the cervical 



