DEVELOPMENT OF EYE 553 



Among the extrinsic structures must be noted the six muscles which 

 move the e3'eball, the upper and lower eyelids, which are often very 

 slightly developed, and the third eyehd 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 becom.es 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 and the muscles of 

 the iris. Meanwhile, surrounding mesoderm has insinu- 

 ated itself past the lens into the cavity of the optic cup, 

 there forming the vitreous humour, while externally the 

 mesoderm also forms the vascular choroid, the firm often 

 cartilaginous sclerotic, the inner layer of the cornea, etc. 

 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 cross one 

 another in a chiasma, and their fibres usually interlace 

 as they cross. 



Alimentary system. — 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, 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 fined 

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

 gut. This region is the fore-gut or stomodseum. Finally, 

 there is usually a slight posterior invagination of ectoderm, 

 forming the anus. The hind-gut or proctodaeum is 

 practically absent in Vertebrates. 



