392 THE BABBIT. 



serving for its nutrition during growth ; it disappears completely 

 when the lens has reached its full size. 



The vitreous body is derived from the mesoblast which grows 

 into the cavity of the optic cup, through the choroidal fissure. 

 It is extremely vascular in the early stages of its formation, 

 receiving its blood from the central artery of the retina. 

 There is no structure in the rabbit corresponding to the pecten 

 of the bird. 



The cornea (Fig. 150, cc) is formed, as in the chick, from 

 mesoblast, which spreads in between the surface epiblast and 

 the lens ; and the anterior chamber of the eye is a space which 

 appears at a rather late stage, between the cornea and the lens. 



The choroid and sclerotic are formed, as in the chick, from 

 the mesoblast surrounding the optic cup. They are developed 

 very late; and on the twenty-first day (Fig. 156), when the 

 cornea is well developed, the choroid and sclerotic are merely 

 represented by a thin layer of connective tissue, devoid of pig- 

 ment. The sclerotic of the rabbit is not cartilaginous at any 

 stage. 



The eyelids are folds of skin, above and below the eyeball. 

 They appear early, and by the fourteenth day have attained 

 some size (Fig. 155, OM, oo). By the nineteenth or twentieth 

 day they have grown completely over the eye, and meet each 

 other along their free edges (cf. Fig. 149) ; a little later (Fig. 

 156, OM, oo) the edges of the two eyelids fuse together, the 

 epidermal layers becoming continuous with each other ; this 

 fusion persists throughout the remaining period of development, 

 and is the cause of the blindness characteristic of the young at 

 birth. 



The third eyelid, or nictitating membrane, is a similar fold of 

 skin, arising at the inner angle of the eye, and lying between 

 the other two eyelids and the eyeball. 



The lacrymal glands arise as solid ingrowths of epiblast into 

 the underlying connective tissue, which subsequently become 

 hollowed out to form the cavities of the glands and ducts. 



