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 sclerotomes, the syndetomes, the myotomes and the dermatomes 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. 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 fourth week, while in the caudal portion it occurs at the end of the fourth week. Cranial and caudal neuropores become progressively smaller until they close completely (by day 26) forming the neural tube. Development of organs and organ systems Organogenesis is the development of the organs that begins during the third to eighth week, and continues until birth. Sometimes full development, as in the lungs, continues after birth. Different organs take part in the development of the many organ systems of the body. Blood Haematopoietic stem cells that give rise to all the blood cells develop from the mesoderm. The development of blood formation takes place in clusters of blood cells, known as blood islands, in the yolk sac. Blood islands develop outside the embryo, on