TIIK NOSE AND EYE. 539 



2. The Eye. 



The mode of development of the human eye is so closely 

 similar to that of the rabbit that it will be needless to describe 

 it in detail. 



The optic vesicles appear as lateral outgrowths of the fore- 

 brain as early as the fifteenth day (Fig. 232, BO). They soon 

 become constricted at their bases, and then doubled up to form 

 the optic cups, in the same manner as in other Vertebrates. 

 Owing to the mode in which this doubling up is effected, a 

 choroidal fissure is left, leading into the cavity of the cup ; and, 

 as in the rabbit, the choroidal fissure extends a little way along 

 the optic stalk, towards the brain. Throughout all the earlier 

 stages of development (Fig. 204), the eye is very small, as in 

 Mammals generally, standing in this respect in marked contrast 

 to the eye of the chick embryo at a corresponding stage of 

 development (cf. Fig. 115). 



The inner wall of the optic cup (cf. Fig. 155) is from the 

 first the thicker of the two ; and by the end of the fourth week 

 is at least four times as thick as the outer wall. 



The lens develops late. In embryos three weeks old it is 

 still an open pit : at four weeks the mouth of the pit has closed 

 (Fig. 204), and from this time the cavity of the lens vesicle 

 becomes rapidly filled up by elongation of the cells forming its 

 inner or deeper wall. During the whole period of its develop- 

 ment, the lens is enveloped in a vascular capsule which serves 

 for its nourishment. New cells are constantly added on round 

 the equator of the lens, and growth continues until the time of 

 birth, when the vascular capsule atrophies and disappears. 



The vitreous body is formed of mesoblast, which makes its 

 way into the cavity of the optic cup through the choroidal 

 fissure : it is very vascular during the earlier stages of its 

 formation. 



The cornea is formed from a layer of mesoblast which grows 

 across the front of the eye, between the outer conjunctival 

 epithelium and the lens, towards the end of the second month. 

 In the deeper part of this layer a cavity appears, which becomes 

 the anterior chamber of the eye. The thick layer of mesoblast 

 in front of this chamber becomes the cornea, while the much 

 thinner layer between the chamber and the lens forms the 



