Nervous System

The central and peripheral nervous systems develop from the neural tube and the neural crest (Trilaminar embryonic disc).

External form

At S10, the neural groove starts to close at the level of the cervical segments. The openings at the two extremities are called rostral and caudal neuropores. The cranial and caudal segments of the neural groove also close progressively, thus forming the neural tube. Closure of the rostral neuropore takes place at S11, and closure of the caudal neuropore at S12. The brain grows larger than the embryonic disc and bends ventrally (Formation of the external form of the body). This curving of the brain occurs in the area of the mesencephalon (mesencephalic flexure). By S12, the curvature has reached 90 degrees. The brain greatly increases in size throughout this period, growing in both ventral and caudal directions. At S13, a new curvature appears, which is the cervical flexure. This flexure marks the boundary between the brain and the spinal cord. By S14, maximum curvature has occurred. The brain is lying on the umbilicus. The mesencephalic and cervical flexures are pronounced. From now on, the head slowly begins to straighten. The growth of the brain goes hand in hand with the development of the skull. The rhombencephalon folds. The pontine flexure begins to appear. It first becomes visible at S14 and is well evident by S17. It subdivides the rhombencephalon into the rostrally located metencephalon (cerebellum and pons) and the caudally located myelencephalon (medulla oblongata). The three large parts of the brain (prosencephalon, mesencephalon and rhombencephalon) can now be distinguished.