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Gastrulation is critical to all organism’s survival. It turns a multicellular organism into fully functioning organs. It occurs about seven hours after fertilization and begins as soon as cleavage is accomplished. The blastula, a single layer of cells, doubles over to form two layers. Each of these layers will play a different role in the formation of the embryo. The formation of the primitive streak marks the start of gastrulation. Mesodermal precursor cells are the origin of the primitive streak. The primitive streak appears when the epiblast begins to thicken. As cells are migrating to form the primitive streak, an indention forms called the primitive groove or the blastopore. Convergent extension allows for the elongation of the primitive streak. Once the primitive streak is formed, the embryo now has true anterior-posterior axis. The anterior region of the primitive streak is referred to as Hansen’s node, which functions are the organizer for the cells. The order that cells enter the blastocoel and through Hansen’s node will determine which of the three germ layers they will become. Cells differentiate and migrate to form the ectoderm, which is the outer layer, the mesoderm, the middle layer, and the endoderm, the inner layer. Ectoderm cells give rise to skin, epidermis, the neural crest and tissue that will eventually form the nervous system. Mesoderm cells will turn into the circulatory system, the kidneys, and skeletal compartments. It will also give rise to somites, which will form muscle, cartilage for the ribs and vertebrae, the dermis, the notochord, blood vessels, and bone for the chick. The endoderm cells give rise to the respiratory and gastrointestinal tracts, such as the liver and pancreas. As the primitive streak begins to descend, Hansen’s node migrates to the posterior region. This will eventually form the anus of the chick (Vasiev et al). As a result of the anterior-posterior division, cells progress through the stages at different rates. Typically, the anterior region is more advanced than the posterior. The mesoderm and endoderm cells migrate inward and surround the yolk by epiboly. No true archenteron is formed during chick gastrulation. As gastrulation comes to an end, Hansen’s node begins to regress and leaves behind the notochord and the ectoderm cells finally migrate and surround the yolk.

Neurulation is the formation of the central nervous system and the development of the neural tube in the ectoderm. Once the the notochord has been formed during late gastrulation, signals are sent to stimulate stem cells in the embryo. This causes the stem cells to orient themselves along the dorsal axis of the chick, which creates the neural plate. The neural plate elevates itself, which forms neural folds. Then invagination of the neural plate creates the neural groove. Before the neural groove completely closes, a group of cells called the neural crest form above the tube. The neural crest contributes to the formation of cranial nerve ganglia and skeleton in the skull. There are two types of neurulation, primary and secondary. Primary neurulation is when neuro-plate cells are directed to be proliferated, invaginated, and pinched off to form a hollow tube. This occurs when the neural groove, which is located in the ectoderm, closes to form the neural tube in the anterior region. As the neural tube closes, it creates the midbrain, forebrain, and hindbrain vesicles. The forebrain will give rise to cerebral hemispheres and optic vesicles. The midbrain will later form the optic receptors for optic nerves and the hindbrain will form the cerebellum and the medulla. Secondary neurulation is when the neural tube is produced by a solid cord of cells that sink into the embryo to form a hollow tube. Like most organisms, chicks preform both primary and secondary neurulation. The anterior portion of the neural tube is formed by primary neurulation. Everything that is posterior to the hind-limbs are made by secondary neurulation. The remainder of the neural tube will form the spinal cord.