ORGANS OF SPECIAL SENSE 



277 



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 lamellae 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 (3) the radial lamellae, which 

 form the greater part of the 

 substance of the adult lens. 

 The meridional rows and the 

 radial lamellae 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 

 lamellaB 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 



FIG. 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.) 



