106 PROF. MARSHALL AND MR, SPENCER. 



root, retains its secondary, and acquires tertiary roots, while the 

 seventh retains both primary and secondary. Concerning the fourth 

 nerve we would suggest the possibility that it may prove to be the 

 primary root of a nerve of which the third nerve is the secondary root, 

 which has, in this case, acquired complete independence. 



c. The branches of the seventh nerve. — At stage K the seventh nerve, 

 which, as we have already seen, has acquired its secondary as well as 

 its primary roots of origin (fig. 3), expands below the secondary root 

 into a large ganglionic swelling lying immediately behind the third 

 head cavity. From this enlargement three branches arise : (1 ) from 

 the anterior and superior angle of the ganglion a large nerve with a 

 ganglionic base arises, and runs forward along the dorsal surface of the 

 head, lying just beneath the superficial epiblast ; this is the ophthalmic 

 branch of the seventh, and is referred to in our figures as vn a. 



(2) The second branch, which is also large, and has a ganglionic 

 base, arises from the front part of the ganglion immediately below the 

 root of the ophthalmic ; its deeper portion runs forwards and slightly 

 downwards over the top of the third head cavity and becomes con- 

 nected with the main stem of the fifth ; it is referred to in the figures 

 as iV.c'. The more superficial portion passes on further forward in 

 the same direction, crosses the mandibular arch, and enters the 

 maxillary process, lying immediately superficial to the maxillary nerve 

 and just beneath the external epiblast ; it is referred to in the figures 

 as vii d. 



(3) The third branch is the direct continuation of the main stem 

 of the facial nerve ; it runs downwards and backwards along the 

 anterior border of the hyoidean arch, and is the rudiment of the 

 posterior or hyoidean branch of the seventh in the adult ; it is referred 

 to in the figures as vii c. 



Of these branches the first, or ophthalmic, is from its earliest 

 appearance connected with the dorsal or primary root of the seventh 

 rather than with the ventral root. The second branch is the most 

 remarkable of the three ; its deeper portion forms, as we have seen, a 

 direct connection between the fifth and seventh nerves, a communi- 

 cation which appears to be very early established, inasmuch as by 

 stage k the connecting branch is already a nerve of considerable size ; 

 the superficial portion of this branch (vii d) is noteworthy, mainly on 



