Development in terrestrial environments:
Reptiles, Aves and mammals develop on land and required protection from desiccation which is provided by extra embryonic membranes. Longer developmental periods of these animals reflect their lack of independent larval stages.
Avian embryology:
Egg: The yellow portion of the chicken egg is single cell produce in chicken ovary. This egg is released into the oviduct where fertilization may occur. Following fertilization, membranes and fluids collect around the egg. A vitelline membrane covers the surface of true egg. The ‘white’ consists of water and a protein called albumen. This watery environment protects the egg from mechanical damage and drying. Albumen is a source of nutrients and is eventually consumed during development.
Two denser strands of albumen (called chalazas) attach to the inside of the shell and to the egg and suspend the egg in the centre of watery albumen. The shell is made up of calcium carbonate impregnated with protein. Thousands of tiny pores in the shell permit gas exchange between the embryo and the outside. On the inside of the shell are two shell membranes. An air pocket forms between these membranes at the rounded end of the shell. The air pocket enlarges during development as air moves through pores in the shell to replace water loss as hatching approaches the chick penetrates the air pocket with its beak, the lungs inflate and the chick begins to breathe from the air sac while still exchanging gases across vascular extra embryonic membranes.
Early cleavages and Gastrulation:
Cleavage of chicken egg is meroblstic. A small disc blastoderm of about sixty thousand cells at the animal end of the egg develops. The blastoderm is raised off the yolk, leaving a fluid filled space analogous to blastocoel of the amphibian blastula. Proliferation and movement of blastoderm cell sort the cells into two layers. The epiblast is the outer layer of cells and the hypoblast is the inner layer. The movements of blastoderm cells are the beginning of gastrulation. The female reproductive tract releases the egg at about this time. A medial, linear imagination called the primitive streak, gradually extends anteriorly. A depression called Henson’s node, forms at the anterior margin of the primitive streak and marks the beginning of inward migration of epiblast cells, comparable to involution of amphibian gastrula.
The primitive streak is therefore analogous toe the dorsal lip of the blastopore. The migration occurs during a dramatic posterior movement of Henson’s node. Migrating cells form mesoderm, what is left of the epiblast forms the endodermal lining of the gut tract. Three germ layers are now arranged above the surface of the yolk.
Following gastrulation, notochordal cells separate from the overlying neural ectoderm and the neural tube forms. In addition mesoderm which originally formed as solid blocks of cells, organizes into somites and splits to form the coelom.
The embryo lifts off the yolk when the margins of the embryo grow downward and meet below the embryo. A connection between the embryo and the yolk is retained and called the yolk stalk. Blood vessels develop in the yolk stalk and carry nutrients from the yolk to the embryo.
Development of extra embryonic membranes:
Extra embryonic membranes of amniotes include the yolk sac, the amnion, the chorion and the allantois. Reptiles and birds have large quantity of yolk that becomes enclosed by yolk sac. Yolk sac develops from proliferation of endoderm and mesoderm around the yolk. The yolk sac is highly vascular and distributes nutrients to the developing embryo.
Amnion and Chorion:
Following the neural tube stage the ectoderm and mesoderm on both sides of the embryo lift off the yolk and grow dorsally over the embryo. As these membranes meet dorsally they fuse and form inner amnion and outer chorion. Amnion encloses the embryo in a fluid filled sac. This amniotic cavity protects against shock and drying. Chorion is nearer the shell, becomes highly vascular and aids in gas exchange.
Allantois:
The immediate breakdown product of proteins is highly toxic ammonia. This ammonia is converted to uric acid which is excreted and stored in the allantois, a ventral outgrowth of the gut tract. Uric acid is semi solid and thus little water is wasted. Allantois gradually enlarges during development to occupy the region between amnion and chorion. In addition allantois become, highly vascular and functions with chorion in gas exchange.
Fate of Mesoderm:
Following gastrulation in birds, reptiles and mammals all three primary germ layers have been formed of the three layers the face of mesoderm is the most complex. Mesoderm forms all of the supportive tissues of vertebrates including connective tissues (bone, cartilage and blood) and muscle. These supportive tissues are frequently associated with derivates of other primary germ layers. For example the inner lining of the gut is endodermal but mesodermally derived structures such as smooth muscles, blood and blood vessels make up the bulk of that system.
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