THE CRANIAL NERVES. 



265 



about half way down its side ; this attachment being effected by 

 the outgrowth of processes from the cells of the nerve, into the 

 substance of the brain. 



This attachment is acquired by the seventh nerve about the 

 end of the second, or early in the third day (Fig. 121, Vii). 

 Owing to the part of the brain dorsal to the nerve growing 

 more rapidly than its ventral part, the root of attachment of the 

 nerve becomes apparently shifted further downwards, towards the 



El 



KM 



RE 



FIG. 120. Transverse section across the head of a Chick Embryo at the forty- 

 third hour of incubation. The section passes through the commencing 

 auditory pits, and through the heart, x 100. 



A, aorta. BH, cavity of hind-brain. CH, notocliord. EL commencing auditory 

 pit. H, hypobList. NB, neural crest ; the section passes through tin- narrow conimis- 

 sural part of the crest, which connects the rudiments of the facial and auditory nerves 

 with those of the gtoflao-pharyngaaJ nerves. HE, endothelial lining of heart. RM, 

 muscular wall of heart. TP, pharynx. 



ventral surface of the brain, and by the end of the third day has 

 acquired the position characteristic of the nerve-root in the adult. 

 At the same time, changes occur in the trunk of the nerve. 

 Owing to intrusion of mesoblast between the surface epiblast 

 and the nerve, the latter becomes more deeply placed than in 

 the early stages. The connection with the sensory patch of the 

 surface epiblast persists, but becomes drawn out, as the nerve 

 recedes from the surface, into a cutaneous branch of greater or 

 less length. Beyond the origin of this cutaneous branch the 

 nerve continues its growth, and by the end of the third day, or 



