278 THE DEVELOPMENT OF THE CHICK 



that the epitheUiim is originall}^ thinnest distally and thickens 

 towards the equator. This condition increases up to the eighth 

 day, at which time the thickening increases more a short distance 

 from the equator, so that there is a broad ring-shaped thickening 

 of the anterior epitheUum separated by a narrow thinner zone 

 from the cells of the equatorial zone (cf. Fig. 159). This ring 

 increases in thickness during the greater part of the period of 

 incubation, and its cells become fibers arranged in a radial direc- 

 tion. The meaning of this curious structure is somewhat obscure, 

 but from the fact that it shows on its surface the impression of 

 the ciliary processes, Rabl was of the opinion that it served in 

 accommodation of the eye as an intermediary between the ciliary 

 processes and the true lens-fibers. 



4. Anterior Chamber and Cornea, etc. When the optic vesicle 

 is first formed it is in immediate contact with the ectoderm. 

 After its invagination the lips of the optic cup withdraw a short 

 distance from the surface. At the same time the lens invagi- 

 nates and is cut off from the ectoderm, but remains in contact 

 with it during the third day. There is thus a ring-shaped space 

 between the lens and optic cup on the one hand and the ectoderm 

 on the other, which is the beginning of the anterior chamber of 

 the eye (cf. Fig. 96 C). With the formation of the cornea the 

 lens withdraw^s somewhat from the surface and the space spreads 

 over the whole external surface of the lens; at first it is very 

 narrow, but increases in size by the formation of the iris and 

 the bulging of the cornea. 



The cornea itself develops from two sources: (1) the external 

 epithelium is derived from the ectoderm overlying the anterior 

 chamber, (2) the cornea propria and the internal epithelium 

 lining the anterior chamber develop from the surrounding mesen- 

 chyme but in somewhat different ways. 



The cornea propria appears on the fourth day as a deli- 

 cate structureless membrane beneath the corneal epithelium. 

 During the fifth day it increases to about the thickness of 

 the overlying ectoderm (Fig. 158). About this time mesen- 

 chyme cells from the margin of the optic cup begin to migrate 

 between the cornea propria and lens, and soon form a single 

 complete layer of cells on the inner face of the cornea propria; 

 this layer becomes the inner epithelium of the cornea (Fig. 158). 

 The cornea propria is still devoid of cells, but on the sixth and 



