xm 



PHYLUM CHORDATA 



109 



ypl.c' 



structure. At an early stage of development a hollow outgrowth 

 the optic vesicle (Fig. 795, A, opt. v.) is given off from each side of 

 the fore-brain (dien.). It extends towards the side of the head, 

 where it meets with an inpushing of the ectoderm (inv. I.) which 

 deepens and forms a pouch, and finally, separating from the ecto- 

 derm, a closed sac (B, I.) with a very small cavity and thick walls. 

 This sac is the rudiment of the lens : as it enlarges it pushes against 

 the optic vesicle, and causes it to become invaginated (B) ; the 

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

 layered optic cup (opt. c., opt. c'.), its cavity, originally continuous 

 with the diacoele, 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 choroid fissure afterwards more or less com- 

 pletely 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 mem- 

 brane, including the 

 rods and cones, is 

 formed. The stalk 

 of the optic 

 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 

 epithelium. 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 



FIG. 795. Early (A) and later (B) stages in the development 

 of the eye of a Craniate, dien. diencephalon ; inv. I. 

 invagination of ectoderm to form lens ; I. lens ; opt. c. outer, 

 and opt. c'. inner layer of optic cup ; opt. st. optic stalk ; 

 opt. v. optic vesicle ; ph. pharynx ; ply. pituitary body . 

 (Altered from Marshall.) 



