ORGANS OF SPECIAL SENSE 
This process is kept up not only 
during embryonic life, but dur- 
ing the entire growth of the 
fowl; thus the thickness of the 
superimposed lamelle is only 
0.60 mm. at hatching, but is 
2.345 mm. in the adult (Rabl). 
In the fowl the lens includes 
three concentric layers of fibers: 
(1) the central mass or core 
formed by the proximal wall of 
the original lens-sac; this has 
the same diameter (0.80 mm.) 
as the entire fiber mass at eight 
days. Nuclei are entirely ab- 
sent. (2) An intermediate layer 
of meridional rows of fibers 
rather irregularly arranged, 
which shade gradually into the 
fibers of the core and into those 
of (8) the radial lamelle, which 
form the greater part of the 
substance of the adult lens. 
The meridional rows and the 
radial lamelle proceed from the 
cells of the intermediate zone 
of the original lens-sac. Fig. 
161 shows a sector of an equa- 
torial section through the lens 
of a chick. The three zones 
are well marked; the extraordi- 
nary regularity of the super- 
imposed layers of the radial 
lamella is well shown. 
The lens epithelium of birds 
and reptiles also produces a 
peculiar structure which may be 
called the equatorial ring (Ring- 
wulst, Rabl). 
It will be seen in the figures 
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Fia. 161.— Equatorial section through 
the lens of a chick embryo of eight 
days. The main mass of the entire 
lens is represented by irregularly 
arranged central fibers. Towards 
the surface (above) the fibers are 
arranged in rows and are quite 
regularly six sided. (After Rabl.) 
