106 ZOOLOGY SECT. 



The mode of development of the eye is as characteristic as its 

 structure. At an early stage of development a hollow outgrowth 

 the optic vesicle (Fig. 741, A, opt. v) is given off from each side of 

 the diencephalon (dien.). It extends towards the side of the head, 

 where it meets with an in-pushing of the ectoderm (inv. I.) which 

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

 ectoderm, a closed sac (B, /.) 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 (), 

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

 layered optic cup (opt. c., opt. c 1 .), its cavity, originally continuous with 

 the diaccele, becoming obliterated. The invagination of the vesicle 

 to form the cup does not take place symmetically, 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 completely closed by the 



in-vl 



FIG. 741. Early (A) and later (B) stages in the development of the eye of a Craniate. 

 dien. diencephalon ; inv. 1. invagination of ectoderm to form lens ; I. lens ; opt. c. outer layer 

 of optic cup ; opt. c'. inner layer ; opt. st. optic stalk ; opt. r. optic vesicle ; ph. pharynx ; 

 pty. pituitary body. (Altered from Marshall.) 



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 epithelium. Mesoderm also makes its way into the optic 

 cup, through the choroid fissure, and becomes the vitreous. Lastly, 

 the mesoderm immediately surrounding the optic cup is differenti- 

 ated 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 



