ORGANS OF VISION OF THE VERTEBRATA. 495 



The development of the zonula of Zinn in Mammalia, which ought to 

 throw some light on the nature of the vitreous humour, has not been fully 

 investigated. According to Lieberkuhn (No. 373, p. 43), this structure 

 appears in half-grown embryos of the sheep and calf. 



He says "At the point where the ciliary processes and the ciliary 

 part of the retina are entirely removed, one sees in the meridian bundles 

 of fine fibres, which correspond to the valleys between the ciliary pro- 

 cesses and fill them ; also between these bundles there extend, as a thin 

 layer, similar finely striated masses, and these would have been on the 

 top of the ciliary processes." He further states that these fibres may be 

 traced to the anterior and posterior limb of the lens-capsule, and that 

 amongst them are numerous cells. Kolliker confirms Lieberkiihn's state- 

 ments. There can be little doubt that the fibres of the zonula are of the 

 nature of connective tissue : they are stated to be elastic. By Lowe they 

 are believed to be developed out of the substance of the vitreous humour, 

 but this does not appear to me to follow from the observations hitherto 

 made. It seems quite possible that they arise from mesoblast cells which 

 have grown into the cavity of the vitreous humour, solely in connection 

 with their production. 



The integral parts of the eye in front of the lens are the 

 cornea, the aqueous humour, and the iris. The development 

 of the latter has already been described, and there remain to be 

 dealt with the cornea, and the cavity containing the aqueous 

 humour. 



The cornea. The cornea is formed by the coalescence of 

 two structures, viz. the epithelium of the cornea and the cornea 

 proper. The former is directly derived from the external epiblast, 

 which covers the eye after the invagination of the lens. The 

 latter is formed in a somewhat remarkable manner, first clearly 

 made out by Kessler. 



When the lens is completely separated from the epidermis 

 its outer wall is directly in contact with the external epiblast 

 (future corneal epithelium). At its edge there is a small ring- 

 shaped space bounded by the outer skin, the lens and the edge 

 of the optic cup. In the chick, which we may take as typical, 

 there appears at about the time when the cavity of the lens is 

 completely obliterated a structureless layer external to the above 

 ring-like space and immediately adjoining the inner face of the 

 epiblast. This layer, which forms the commencement of the 

 cornea proper, at first only forms a ring at the border of the 

 lens, thickest at its outer edge, and gradually thinning off to 



