496 



THE CORNEA. 



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 surround- 

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

 layer of the membrane of Descemet. After it 1 has become 



FIG. 290. SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAY 

 OF DEVELOPMENT, TO SHEW THE IRIS AND CORNEA IN THE PROCESS OF 

 FORMATION. (After Kessler.) 



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

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



completely established, the mesoblast around the edge of the 

 cornea becomes divided into two strata ; an inner one (fig. 290, 

 cb) 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 posterior (Descemet's membrane) of the cornea. The corneal 



1 It appears to me 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 correction. From the structure of the eye in the Ammoccete it 

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



