<P> The three germ layers are the ectoderm, mesoderm and endoderm, and are formed as three overlapping flat discs . It is from these three layers that all the structures and organs of the body will be derived through the processes of somitogenesis, histogenesis and organogenesis . The embryonic endoderm is formed by invagination of epiblastic cells that migrate to the hypoblast, while the mesoderm is formed by the cells that develop between the epiblast and endoderm . In general, all germ layers will derive from the epiblast . The upper layer of ectoderm will give rise to the outermost layer of skin, central and peripheral nervous systems, eyes, inner ear, and many connective tissues . The middle layer of mesoderm will give rise to the heart and the beginning of the circulatory system as well as the bones, muscles and kidneys . The inner layer of endoderm will serve as the starting point for the development of the lungs, intestine, thyroid, pancreas and bladder . </P> <P> Following ingression, a blastopore develops where the cells have ingressed, in one side of the embryo and it deepens to become the archenteron, the first formative stage of the gut . As in all deuterostomes, the blastopore becomes the anus whilst the gut tunnels through the embryo to the other side where the opening becomes the mouth . With a functioning digestive tube, gastrulation is now completed and the next stage of neurulation can begin . </P> <P> Following gastrulation, the ectoderm gives rise to epithelial and neural tissue, and the gastrula is now referred to as the neurula . The neural plate that has formed as a thickened plate from the ectoderm, continues to broaden and its ends start to fold upwards as neural folds . Neurulation refers to this folding process whereby the neural plate is transformed into the neural tube, and this takes place during the fourth week . They fold, along a shallow neural groove which has formed as a dividing median line in the neural plate . This deepens as the folds continue to gain height, when they will meet and close together at the neural crest . The cells that migrate through the most cranial part of the primitive line form the paraxial mesoderm, which will give rise to the somitomeres that in the process of somitogenesis will differentiate into somites that will form the sclerotome, the syndetome, the myotome and the dermatome to form cartilage and bone, tendons, dermis (skin), and muscle . The intermediate mesoderm gives rise to the urogenital tract and consists of cells that migrate from the middle region of the primitive line . Other cells migrate through the caudal part of the primitive line and form the lateral mesoderm, and those cells migrating by the most caudal part contribute to the extraembryonic mesoderm . </P> <P> The embryonic disc begins flat and round, but eventually elongates to have a wider cephalic part and narrow - shaped caudal end . At the beginning, the primitive line extends in cephalic direction and 18 days after fertilization returns caudally until it disappears . In the cephalic portion, the germ layer shows specific differentiation at the beginning of the 4th week, while in the caudal portion it occurs at the end of the 4th week . Cranial and caudal neuropores become progressively smaller until they close completely (by day 26) forming the neural tube . </P>

When does neurulation occur in the course of human development
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