VI.] THE CORNEA. 151 



in contact with the epidermis (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. 

 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 epidermis. 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 

 away towards the centre. It soon however becomes 

 broader, and finally forms a continuous stratum of con- 

 siderable thickness, interposed between the external 

 skin and the lens. As soon as this stratum has 

 reached a certain thickness, a layer of flattened cells 

 grows in along its inner side from the mesoblast sur- 

 rounding the optic cup (Fig. 52, dm). This layer is 

 the epithelioid layer of the membrane of Descemet 1 . 

 After it has become completely established, the meso- 

 blast around the edge of the cornea becomes divided 

 into two strata ; an inner one (Fig. 52 cb) destined to 

 form the mesoblastic tissue of the iris already described, 

 and an outer one (Fig. 52 cc) adjoining the epidermis. 

 The outer stratum gives rise to the corneal corpuscles, 

 which are the only constituents of the cornea not yet 

 developed. The corneal corpuscles make their way 



1 It appears possible that Lieberkiihn may be right in stating 

 that the epithelium of Descemet 's membrane grows in between the 

 lens and the epiblast before the formation of the cornea proper, and 

 that Kessler's account, given above, may on this point require correc- 

 tion. From the structure of the eye in some of the lower forms it 

 seems probable that Descemet's membrane is continuous with the 

 choroid. 



