XIII 



PHYLUM CHORDATA 



113 



the single-layered optic vesicle thus becomes converted into a two- 

 layered optic cup {opt. c, opt. ci.), its cavity, originally continuous with 

 the diaccBle, becoming obliterated. The invagination of the vesicle 

 to form the cup does not take place symmetrically, but obliquely from 

 the external (posterior) and ventral aspect of the vesicle, so that 

 the optic cup is incomplete along one side where there is a cleft— 

 the choroicl fissure — afterwards more or less completely closed by the 

 union of its edges. The outer layer of the optic cup becomes the 

 pigmentary layer of the retina : from its inner layer the rest of that 

 membrane, including the rods and cones, is formed. The stalk of 

 the optic cup occupies, in the embryonic eye, the place of the optic 

 nerve, but the actual fibres of the nerve are formed as backward 

 growths from the nerve-cells of the retina to the brain. 



During the formation of the lens, mesoderm grows in between 

 the pouch from which it arises 

 and the external ectoderm ; 

 from this the main substance 

 of the cornea and its inner 

 or posterior epithelium are 

 formed, the adjacent ectoderm 

 becoming the external epi- 

 thelium. Mesoderm also makes 

 its way into the optic cup, 

 through the choroid fissure, 

 and becomes the vitreous 

 humour. Lastly, the mesoderm 

 immediately surrounding the 

 optic cup is differentiated to 

 form the choroid, the iris, and 

 the sclerotic. 



Thus the paired eye of 

 Vertebrates has a threefold 

 origin : the sclerotic, choroid, 



iris, vitreous, and the greater part of the cornea are mesodermal : 

 the lens and external epithelium of the cornea are derived 

 from the ectoderm of the head : the retina and optic nerve are 

 developed from a hollow pouch of the brain, and are therefore, 

 in their ultimate origin, ectodermal. The sensory cells of the 

 retina — the rods and cones, although not directly formed from 

 the external ectoderm, as in Invertebrates, are ultimately traceable 

 into the superficial layer of ectoderm, since they are developed 

 from the inner layer of the optic vesicle, which is a prolongation 

 of the inner layer of the brain, and the latter is continuous 

 before the closure of the medullary groove with the ectoderm 

 covering the general surface of the body. 



The eye-ball is moved by six muscles (Fig. 787). Four of these 

 arise from the inner wall of the orbit, and pass, diverging as they 



Fig. 787, a. — Musclea And iieivesof the e>L of 

 , a skate (semi diagiamxuatic) III ociilo- 

 ' motor nerve, 7r, trochleai , VI, abducent. 

 c. r. external rectu.s ; I. o. inferior oblique ; 

 in. r. inferior rectus ; i. r. internal rectus ; 

 or. wall of orbit ; s. o. superior oblique ; s. r. 

 superior rectus. 



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