448 STRUCTURE OF VERTEBRATA. 



this there lies the most sensitive region the yellow spot, 

 with its fovea centralis, where all the layers of the retina 

 have thinned off except the cones. 



Among the extrinsic structures, must be noted the six muscles which 

 move the eyeball, the upper and lower eyelids which are often very 

 slightly developed, and the third eyelid or nictitating membrane. 

 Above Fishes there is a lachrymal gland associated with the upper lid, 

 and a Harderian gland associated with the nictitating membrane. In 

 Mammals there are also Meibomian glands. The secretions of all these 

 glands keep the surface of the eye moist. 



While the medullary groove is still open, the eyes arise 

 from the first vesicle of the brain as hollow outgrowths or 

 primary optic vesicles. Each grows till it reaches the skin, 

 which forms a thickened involution in front of it. This 

 afterwards becomes the compact lens. Meantime it sinks 

 inwards, and the optic vesicle becomes invaginated to form 

 a double walled optic cup. The two walls fuse, and the one 

 next the cavity of the cup becomes the retina, while the 

 outer forms the pigmented epithelium. Meanwhile sur- 

 rounding mesoblast has insinuated itself past the lens into 

 the cavity of the optic cup, there forming the vitreous 

 humour, while externally the mesoblast also forms the 

 vascular choroid, the firm often cartilaginous sclerotic, the 

 inner layer of the cornea, &c. Along the thinned stalk of 

 the optic cup the optic nerve is developed. Its protective 

 sheath is continuous with the sclerotic of the eye and the 

 dura mater of the brain. As the nerves enter the optic 

 thalami, they always cross one another in a chiasma, and 

 their fibres usually interlace as they cross. 



The Alimentary System and Associated Structures. 



The alimentary tract exhibits much division of labour, 

 for not only are there parts suited for the passage, digestion, 

 and absorption of the food, but there are numerous out- 

 growths, e.g., lungs and allantois, which have nothing to do 

 with the main function of the food canal. 



By far the greater part of the food canal is lined by 

 endoderm or hypoblast, and is derived from the original 

 cavity of the gastrula the primitive gut or archenteron. 

 This is the mid gut or mesenteron. But the mouth cavity 

 is lined by ectoderm, invaginated from in front to meet the 



