278 THE DEVELOPMENT OF THE CHICK 
that the epithelium is originally 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 epithelium 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 (ef. Fig. 96 C). With the formation of the cornea the 
lens withdraws 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 
