Continued from page 2The Egg
The egg consists of a germinal disk, yolk, the membranes surrounding the yolk, albumen and a shell.
The germinal disk (blastoderm if fertilized, blastodisc if unfertilized) is a small disk of cytoplasm that can be seen on the surface of the yolk of a fresh egg as a circular, opaque white spot - approximately 3-4 mm in diameter in the domestic fowl.
The yolk is a thick viscous material containing about 50% solids. Of these solids, 99% are proteins (30% lipoproteins and phosphoproteins). Yolk provides the main source of nutrition for the developing embryo. There are two kinds of yolk - white and yellow. The white yolk is approximately 2/3 protein and 1/3 fat. The central area of white yolk in the ovum is called the latebra. The yellow yolk is approximately 2/3 fat and 1/3 protein. Yellow yolk can be laid down in alternating whitish and yellow strata, depending on diet of the hen - carotenoid pigments providing the yellow color. An ovum from a hen with a well balanced diet will display no stratifications in yellow yolk coloration.
The yolk membranes form a barrier between the yolk and albumen which provides good mechanical strength but is permeable to water and salts. The yolk membranes consist of four layers from closest to the yolk: the Oocyte laminella and the Perivitelline lamina (derived from the follicle of the ovary) and the Continuous lamina and the Extravitelline lamina (from the oviduct soon after the egg has entered).
The chalaziferous layer is a thin layer of dense albumen which encloses the yolk membranes. The two chalazae are twisted strands of fine ovomucin fibers formed from the chalaziferous layers, and are secreted by the tubular portion of the infundibulum. The chalazae are continuous with the chalaziferous layer and the dense albumen layer - forming a suspension device for suspending the ovum in the center of the egg. The albumen is much less viscous than the yolk. The solid component (dense albumen) of albumen is composed almost entirely of protein (ovomucin) and some mucin fibers. Thin albumen is more watery and contains less ovomucin and almost no mucin fibers. Albumen is secreted by the magnum and contributes to the aqueous environment of the embryo, has anti-bacterial properties, and in many if not all birds provides a source of nutrition for the developing embryo to consume.
The shell consists of the shell membranes, the testa and the cuticle. The inner and outer shell membranes separate during the cooling phase following oviposition at the blunt end of the egg - allowing for the formation of the air cell. The outer shell membrane is firmly attached to the testa, and the inner shell membrane is fused to the albumen ligament of the dense layer of albumen. The testa is composed of an organic matrix and has an inorganic component as well. The inner organic matrix is termed the mamillary layer, and the outer and thicker organic matrix is termed the spongy layer. The mamillary cones are embedded into the outer shell membrane. The cuticle is a continuous layer of organic material (90% peptide) overlaying the testa, and provides protection from water penetration, reduces water losses, and acts as a barrier to bacteria. Most pigmentation of the egg occurs in the inorganic layers of the testa or in the cuticle.
Incubation
The Developing Egg and Embryo
The embryo is formed from a fertilized blastoderm or germinal spot on the ovum. Females carry the sex linked chromosome and are responsible for determining the sex of the avian embryo. Each ovum carries either a Z or W chromosome, wheras the male is ZZ, and his spermatozoa can contribute only a Z chromosome.
Three layers of membranes protect and segregate the developing embryo. The extra embryonic membranes consist of two layers - the ectoderm and mesoderm, or endoderm and mesoderm, depending on the interface of the individual membrane. As the blastoderm matures, the amnion grows out around the developing embryo to form the sac containing the amniotic fluid. The embryo develops suspended in this amniotic fluid, which is in turn surrounded by the amniotic membrane. The second concentric membrane, the chorion, expands to line the inner shell wall. The third membrane, the allantois, develops from the hind gut of the embryo and also lines the inner shell. The combined membranes are called the chorioallantois, and this highly vascularized surface acts as both respiratory and excretory systems for the embryo. Blood vessels of the chorioallantois carry oxygen from the shell lining to the embryo and bring carbon dioxide back to the surface. This inner shell lining then acts like a large, passive lung. Metabolic wastes are deposited in part as urate crystals within the allantois. The insoluble urates are the most biologically inert from of nitrogenous wastes. If birds, like mammals, excreted nitrogenous wastes as ammonia (urea), they could not reproduce by laying eggs: the toxic ammonia within the egg would quickly prove to be toxic to the developing embryo.
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