276 THE DEVELOPMENT OF THE CHICK 
3. The Lens. The account of the development of the lens is 
mainly after Rabl. The wall of the lens-sac is everywhere a sin- 
ele-layered epithelium, though the nuclei are at different levels in 
the cells. 
Shortly after the lens-sac has become separated from the ecto- 
derm the proximal wall (that next the cavity of the optic cup) 
begins to thicken by elongation of the constituent epithelial cells 
(Figs. 157 and 158). During the fourth day the elongation of the 
cells increases greatly as the first step in the formation of the lens 
fibers, while those of the distal wall remain practically unchanged, 
being destined to form the epithelium of the lens. Between the 
cells of the proximal and distal walls are found cells of an inter- 
mediate character, bounding the equator of the lens (Fig. 158). 
During the fifth day the elongation of the cells of the proximal 
wall proceeds apace; those in the center of the wall are most 
elongated and there is a gradual decrease towards the equator 
of the lens. In this way the face of the proximal wall gradually 
approaches the distal wall and meets it on the fifth day, thus 
obliterating the central part of the lens cavity, though the periph- 
eral part remains open for a considerably longer time (Fig. 158). 
The nuclei of the lens fibers occupy approximately their center, 
and thus form a fairly broad curved band, concave towards the 
optic cup. At the same time the lens is increasing very rapidly 
in size. 
During the sixth, seventh, and eighth days the same processes 
continue and the elongation of the lens fibers makes itself felt 
on the inner face of the lens which becomes convex. The form 
and arrangement of the parts is shown in Figure 159. The fibers 
already present are destined to form only the core of the adult 
lens; and a new process begins at this time, leading to the forma- 
tion of fibers that wrap themselves around this core in a merid- 
ional direction and form many concentric layers (666 according 
to Rabl). These new concentric fibers proceed from cells situated 
between the core fibers and the lens epithelium, that is, around 
the equator of the lens. There is a very rapid multiplication of 
cells here; those next the core transform into fibers arranged 
meridionally on the surface of the core; others develop over these 
and thus the original fibers come to be surrounded by more and 
more concentric layers. At first these are disposed rather irregu- 
larly, but soon the arrangement becomes extraordinarily regular. 
