Neurulation

A cross-section of a gastrula shows the tropoblast (forms chorion and placenta), endoderm (epithelial layer of digestive tract, lungs, liver, pancreas, thyroid, parathyroid, thymus), mesoderm (muscular, skeletal, cardiovascular, excretory, reproductive systems), and ectoderm (skin, nervous system).

Before the ectoderm forms the nervous system, the notochord, a cylindrical rod of mesodermal cells, must form. It runs the entire length of the embryo and stimulates the thickening of the ectoderm to produce the neural plate.

Gastrulation forms the three germ layers (ectoderm, mesoderm, endoderm) that produce all structures and systems in a human adult. The nervous system is the first system produced, a process called neurulation, where the ectoderm forms the nervous system.

The notochord induces the ectoderm's neural plate to invaginate, forming the neural groove and neural folds. Continuous invagination brings the neural folds together to form the neural tube and neural crests. The neural tube develops into the central nervous system (brain and spinal cord), while the neural crests form the peripheral nervous system (ganglia).

Neurulation is the process of forming the neural tube and neural crests, which give rise to the central and peripheral nervous systems, respectively. This critical developmental step occurs shortly after the formation of the ectodermal layer during gastrulation.