. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 mm. at hatching, but is mm. in the adult (Ral)l). 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 ( mm.) as the entire fiber mass at eight days. Nuclei are entirely ab- sent. (2) An intermed
. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 mm. at hatching, but is mm. in the adult (Ral)l). 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 ( 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 lamellae 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.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lillie, Frank Rattray, 1870-1947. New York, Holt
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