. The development of the chick; an introduction to embryology. Birds -- Embryology. EMBRYO AND EMBRYONIC MEMBRANES 227 vessels and the folds of the yolk-sac, which will be considered more fully bevond. The interior of the yolk-sac is lined with entoderm which differs in its structure in different regions: In the area pellucida the cells are flattened; in the vascular zone of the area opaca are found the columnar cells with swollen ends described pre- viously. After the third or fourth day these are found filled with yellow fatty droplets, which give a yellow tone to the interior of the living
. The development of the chick; an introduction to embryology. Birds -- Embryology. EMBRYO AND EMBRYONIC MEMBRANES 227 vessels and the folds of the yolk-sac, which will be considered more fully bevond. The interior of the yolk-sac is lined with entoderm which differs in its structure in different regions: In the area pellucida the cells are flattened; in the vascular zone of the area opaca are found the columnar cells with swollen ends described pre- viously. After the third or fourth day these are found filled with yellow fatty droplets, which give a yellow tone to the interior of the living yolk-sac, and which are so abundant in later stages as to render the layer perfectly opaque. These cells do not con-. FiG. 132. — Part of the interior of the yolk-sac of a duck at the time of hatchng. In the upper part of the figure the septa are seen from the side showing the stomata. In the lower part they are seen on edge. Note the sinuous course of the arteries along the free edges of some of the septa. (After H. Virchow.) tain entire yolk-granules; apparently, then, the yolk-granules are digested l^efore absorption in this region. In the region of the inner zone of the vitelline area, the entoderm is composed of several layers of large cells containing yolk-granules, and in the outer vitelline zone we come to the germinal wall. The germinal wall and inner zone of the vitelline area represent the formative region of the yolk-sac epithelium in the manner already described (Chap. V). Blood-vessels of the Yolk-sac. The development of the circu-. 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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