THE EYE AND EAK. 139 



nerve-fibres ; but shortly after this period (Fig. G6) certain of 

 the epithelial cells at the inner surface of the optic cup become 

 pyriform in shape, forming what are termed neuroblasts. From 

 the narrower ends of the neuroblasts, nerve-fibres grow out which 

 spread over the ventral edge of the optic cup, and grow back as 

 a bundle of nerve-fibres along the ventral and posterior wall of 

 the optic stalk, and towards the brain. The optic stalk itself ap- 

 parently takes no direct part in the formation of the nerve-fibres ; 

 its cavity becomes obliterated shortly after the mouth opening 

 is established, except at the end next the brain, where the 

 cavity persists, as the optic recess, throughout life. The rest of 

 the stalk gradually becomes broken up, as the distance between 

 the brain and the eye increases with growth of the tadpole. 



The optic fibres reach the under surface of the brain shortly 

 after the mouth opens, and cross over almost at once to the 

 opposite side of the brain to form the optic chiasma. 



The outer coats of the eye, choroid, sclerotic, and cornea, are 

 formed from the mesoblast surrounding the optic cup. 



The eye develops very slowly, and during the greater part 

 of the tadpole stage of existence is in an imperfect condition ; at 

 the time of the metamorphosis it moves nearer to the surface, 

 and becomes a functionally more perfect organ. 



3. The Ear. 



General account. The ears are developed as a pair of pit- 

 like imaginations of the deeper or nervous layer of the epiblast, 

 at the sides of the hind-brain. The invaginations do not involve 

 the epidermic or outer layer of the epiblast, which is continued 

 across the mouths of the pits. The auditory pits, therefore, do 

 not, in the frog, open at any time to the exterior. 



The mouths of the pits very early narrow and close, and the 

 auditory vesicles so formed separate completely from the epiblast, 

 and lie imbedded in the mesoblast at the sides of the head. By 

 folding of its walls, and by the ingrowth of septa, the vesicle, 

 from being a simple, almost spherical sac, becomes divided up 

 into the complicated auditory vestibule of the adult. 



The auditory nerve becomes connected with the inner wall 

 of the vesicle at a very early stage, indeed almost from its first 

 appearance ; the relations of the nerve to the wall of the vesicle 

 being essentially similar to those between the other cranial 



