THE SENSE ORGANS 



413 



connective tissue surrounding the eye-ball, all come from the mesenchyma. 

 The tarsal and lacrimal glands associated with the eyelids are ectodermal. 

 The optic vesicles arise as lateral outpocketings of the fore-brain. In 

 some animals, however, such as the elasmobranchs and amphibians, the 

 anlagen of the optic vesicles appear primarily as paired depressions of 

 the neural plate. As the optic vesicles continue their lateral expansion, 



Fig. 363. — Sections of successive stages in the development of the lens of the eye 

 from the first thickening of the ectoderm (ec) to the complete separation of the lens, I. 

 (From Kingsley's "Comparative Anatomy of Vertebrates.") 



the connexion with the brain becomes constricted to an optic stalk, which 

 later is converted into a shallow trough to guide the fibers of the optic 

 nerve as they grow from the retina to the brain wall. 



Where the optic vesicle, as it expands, comes into contact with the 

 ectoderm, a local thickening of the ectoderm forms as the anlage of the 

 lens. This placode thickens, invaginates, and sinks below the surface to 

 form a small hollow vesicle, which eventually loses connexion with the 



CONJUNCTIVA- 



AQUEOUS HUMOR- 



VITREOUS BODY 



Fig. 364. — Diagram illustrating the growth of optic nerve fibers from the retina 

 along the optic stalk into the brain. Some of the fibers cross below the brain to form 

 the optic chiasma, while the remaining pass into the thalamus of the same side 



skin. The lens vesicle is finally converted into a solid lens by the thicken- 

 ing of its medial wall, the epithelial cells of which become elongated into 

 fibers and arranged in layers like the coats of an onion. 



While the lens is undergoing these changes, it becomes enclosed by 

 the optic cup, formed by invagination of the optic vesicle. The optic 

 vesicle is thus converted into a two-layered optic cup, attached to the 

 brain by the optic stalk. The cup is, however, incomplete, for a fissure, 



