42 THE EMBRYOLOGY OF THE EYE. 



shorter, and the cells remain as fixed corneal 

 corpuscles between the lamellse of fibres. 



The external epithelium for a considerable 

 time consists of a single stratum of cuboidal 

 cells (Figs. 6 and 7). Then a stratum of flat- 

 tened cells appears external to the cuboidal 

 layer (Fig. 8). From this time on, the epi- 

 thelium gradually thickens, the cells of the 

 external layers remaining flat, those of the 

 middle layers irregularly cuboidal, while those 

 of the inner layers are columnar. Just beneath 

 Bowman's membrane vessels are seen near the 

 periphery of the cornea, continuous with con- 

 junctival vessels. 



6. Sclera. — At the time when the lens is beinor 



o 



formed, the mesoblastic tissue outside the sec- 

 ondary optic vesicle consists of a mass of con- 

 nective-tissue cells of various forms, the nucleus 

 being round or nearly so, and the cells having 

 no particular arrangement. Scattered through 

 this tissue are thin-walled blood-vessels, and a 

 network of these vessels lies on the vesicle- 

 wall (Figs. 3, 6). A little later these vessels 



