VI.] THE CRANIAL NERVES. 147 



8. Daring the latter half of the fourth day there appears 

 at the bottom of the deep lozenge-shaped cavity of the mouth, 

 in the now thin wall dividing it from the alimentary canal, a 

 longitudinal, or according to Gotte a vertical slit which, soon 

 becoming a wide opening, places the two cavities in complete 

 communication. 



The cavity of the mouth, being, it will be remembered, 

 formed partly by depression, partly by the growth of the sur- 

 rounding folds, is lined entirely with epiblast, from which the 

 epithelium of its surface and of its various glands is derived. 

 In this respect, as Remak pointed out, it differs from all the 

 rest of the alimentary canal, whose whole epithelium is formed 

 out of hypoblast. 



9. By the side of the hind-brain, in which the cerebellum, 

 through the increased thickening of its upper walls, is be- 

 coming' more and more distinct from the medulla oblongata, 

 both in front and behind the auditory vesicle, in which the 

 rudiments of the cochlea and recessus vestibuli 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. 46, 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 (Fig. 46, VII.) nerve, 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. 46, G. Ph.), 

 and the hinder one already shews itself to be the pneumo- 

 gastric (Fig. 46, Pg.). 



10. Besides the progressive changes of the alimentary 

 canal and its surroundings, which we incidentally described 

 in the last chapter, and the closure of the mid-gut to form the 

 umbilical duct, of which we have also already spoken, a totally 

 new and most important appendage of the digestive tract, 

 the allantois, becomes for the first time conspicuous on this 

 day, though the first rudiments of it appeared on the third. 



Soon after its appearance the allantois may easily be recog- 

 nized as a pear-shaped vesicle lying in the hinder district of 

 the pleuroperitoneal space, and connected with the under 



10—2 



