46 . DEVELOPMENT OF THE DOG-EISH. 



impressions, and have branches of the olfactory nerve going to 

 them. A capsule is formed round each for protection, which 

 fuses with the brain-box. 



AUDITOEY OKGANS. 



On each side of posterior part of head a dimple appears, which 

 deepens into a sac, whose external opening becomes constricted. 

 The sac then gives off three projections in different directions; these 

 become compressed, and central portion absorbed, leaving three 

 canals, the semicircular canals, each of which of course com- 

 municates with the primary sac, which becomes the vestibule. A 

 cartilaginous capsule is formed around it for protection, which 

 fuses with the brain-box. 



EYE. 



Commences as a lateral outgrouth of anterior cerebral vesicle, 

 in form of a stalked vesicle. The stalk becomes narrower and 

 subsequently solid, and converted into optic nerve. This optic 

 nerve gets pushed backwards, so as to be connected with middle 

 cerebral vesicle, and some of the fibres get pushed over to opposite 

 side, forming the optic commissure. 



The optic vesicle is pushed outwards until it comes in contact 

 with the skin (skin — superficial epiblast). The part of skin with 

 which it comes in contact thickens ; the thickened portion is then 

 pushed inwards in form of a shallow pit with thick walls, carrying 

 before it the front wall of the optic vesicle. To such an extent 

 does the involution of the skin take place, that' the front wall of 

 the optic vesicle is pushed close up to the hind wall, and the 

 cavity of the vesicles becomes almost obliterated. The bulb of 

 the optic vesicle is thus converted into a cup with double walls, 

 containing in its cavity a portion of the involuted skin. We 

 may call this cup the optic cup of its double walls — inner or 

 anterior is formed from the front portion, the outer or pos- 

 terior form the hind portion of wall of the primary optic vesicle. 

 The inner or anterior, which very speedily becomes thicker than 

 the other, is converted into the retina; in the outer, or posterior, 

 which remains, thin pigment is eventually deposited, and it 



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