408 THE CORNEA. 



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 com- 

 mencement 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 nothing towards the centre. It soon however becomes broader, 

 and finally forms a continuous stratum of considerable 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 surrounding the 

 optic cup (fig. 290, dm). This layer is the epithelioid layer of the 



Fig. 290. Section through the eye of a Fowl on the eighth day of develop- 

 ment, TO shew the ikis and cornea in the process of formation, (After Kessler.) 



BT^. epiblastic epithelium of cornea ; cc. corneal corpuscles growing into the struc- 

 tureless matrix of the cornea; dm. Descemet's membrane; ir. iris; ch. mesoblast of 

 the iris (this reference letter points a little too high). 



The space between the layers dm. and ep. is filled with the structureless matrix of 

 the cornea. 



membrane of Descemet. After it^ has become completely esta- 

 blished, the mesoblast around the edge of the cornea becomes divided 

 into two strata; an inner one (fig. 290 ch) destined to form the 

 mesoblastic tissue of the iris already described, and an outer one 

 (fig. 290 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 through the 

 structureless corneal layer, and divide it into two strata, one adjoining 

 the epiblast, and the other adjoining the inner epithelium. The two 

 strata become gradually thinner as the corpuscles invade a larger 

 and larger portion of their substance, and finally the outermost portion 

 of them alone remains as the membrana elastica anterior and pos- 

 terior (Descemet's membrane) of the cornea. The corneal corpuscles, 

 which have grown in from the sides, thus form a layer which becomes 

 continually thicker, and gives rise to the main substance of the cornea. 



1 It appears to me possible that Lieberkiihu 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 correction. From the structure of the eye in the Ammocoete it 

 seems probable that Descemet's membrane is continuous with the choroid. 



