VII.] THE CRANIAL NERVES. 203 



of the stomodseum or primitive buccal cavity, in the now 

 ; thin wall dividing it from the alimentary canal, a longi- 

 I tudinal, or according to Gotte a vertical slit which, soon 

 I becoming a wide opening, places the two cavities in 

 I complete communication. 



The cavity of the mouth, being, it will be remember- 

 ed, formed partly by depression, partly by the growth 

 of the surrounding folds, is lined entirely with epiblast, 

 from which the epithelium of its surface and of its 

 various glands is derived. In this respect, as Kemak 

 jjointed out, it differs from all the rest of the alimentary 

 canal, whose whole epithelium is formed out of hypoblast. 

 By the side of the hind-brain the cerebellum, through 

 the increased thickening of its upper walls, is becoming 

 more and more distinct from the medulla oblongata; 

 while both in front and behind the auditory vesicle, 

 in which the rudiments of the cochlea and recessus ves- 

 tibuli are already visible, the cranial ganglia and nerves 

 are acquiring increased distinctness and size. They may 

 be very plainly seen when the head of the fresh embryo 

 is subjected to pressure. 



The foremost is the fifth cranial nerve (Fig. G7, V.) 

 with its Gasserian ganglion; it lies a little way behind 

 the anterior extremity of the notochord immediately 

 , below the cerebellum. Next to this comes the seventh 

 nerve (Fig. 67, VII.), starting just in front of the ear- 

 vesicle, and extending far downwards towards the second 

 visceral arch. The two nerves which lie behind the ear- 

 vesicle are now obviously separate from each other; the 

 front one is the glossopharyngeal (Fig. 67, G.Ph.), and 

 the hinder one already shews itself to be the pneumo- 

 |i gastric (Fig. 67, Pg.). 



