THE ALIMENTARY CANAL. 389 



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 into 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, etc. The thipned stalk of the optic cup 

 persists as the optic nerve, whose protective sheath is contin- 

 uous 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 outgrowths, 

 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 we shall call the mid-gut, using occasionally the more 

 technical name mesenteron. But the mouth cavity is lined 

 by ectoderm, invaginated from in front to meet the mesen- 

 teron. This region we shall call fore-gut, using occasionally 

 the technical term stomatodseum. Finally, there is usually 

 a slight posterior invagination of ectoderm, forming the 

 anus. This we shall call the hind-gut, occasionally using 

 the technical term proctodseum. It should be noted, that 

 many anatomists use the terms fore-gut, mid-gut, hind-gut, 

 for divisions of the mesenteron, but this contradicts a usage 



