The neural folds have fused in the midline throughout the length of the embryo forming the neural tube on the dorsal side from head to tail. The overlying ectoderm is adjacent to the tube.
The cranial portion of the neural tube begins to enlarge to form the brain leaving the long, caudal portion to become the spinal cord. The rostral wall of the tube, where the rostral neuropore closes, remains thin and constitutes the lamina terminalis.
The neural tube becomes C shaped with two sharp ventral bends, one in the midbrain area, the cephalic flexure, the other at the brain-spinal cord junction, the cervical flexure.
The layer of proliferating neuroepithelial cells around the lumen of the neural tube is called the ependymal layer. Peripheral to this layer, neuroblasts collect in a middle layer called the mantle layer. Neuroblastic processes begin to collect in the periphery of the tube producing a relatively clear zone called the marginal layer.
BRAIN
After the neural folds fuse, the brain is subdivided into three dilations or vesicles named the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). The cranial end of the prosencephalon is called the lamina terminalis.
The lumen of each vesicle forms a ventricle called the prosocoele, mesocoele and rhombocoele, respectively. The roof of the rhombocoele is very thin.
The optic vesicle develops as an outpouching that allows for further subdividing of the prosencephalon into telencephalon and diencephalon. The optic vesicle is considered part of the diencephalon.
Shallow grooves appear in the lateral wall of the brain separating its subdivisions into various portions.
Diencephalic part of prosencephalon—The sulcus medius separates the region of the dorsal thalamus from the ventral thalamus and is continuous caudally with the sulcus limitans. The epithalamus develops in the roof, in front of the pretectum. The sulcus ventralis separates the ventral thalamus from the hypothalamus and is continuous rostrally into the optic stalk. That part of the hypothalamus that is adjacent to the hypophyseal pouch is the neurohypophyseal bud area. The more rostral part where the optic stalk attaches is the optic chiasmal area.
Mesencephalon—The sulcus limitans divides the wall longitudinally into a dorsal part, the tectum, and a ventral part, the tegmentum. The basis pedunculi area will form ventral to the tegmentum.
Rhombencephalon—A series of vertical grooves segment its ventrolateral wall into six rhombomeres (neuromeres). This segmentation is partially evident on the outside of the tube as well. In the caudal portion the sulcus limitans separates the rhombencephalon into dorsolateral (alar plate) and ventrolateral (basal plate) areas.
SPINAL CORD
The long, caudal segment of the neural tube becomes the spinal cord. It gradually tapers in the tail region where it terminates as the primitive filum terminale.
The wall is thin in the midline dorsally and ventrally forming roof and floor plates, respectively. These areas will serve as pathways for crossing nerve fibers. The sulcus limitans divides the mantle layer in the lateral wall into a) a dorsolateral portion called the alar plate in which sensory nuclei form and b) a ventrolateral portion called the basal plate in which motor nuclei form.
PERIPHERAL NERVOUS SYSTEM
Neural crest cells become abundant in the angle between the dorsolateral wall of the neural tube and the surface ectoderm.
CRANIAL NERVES
Cranial Neural Crests
The neural crest cells along the cranial part of the neural tube collect into columns that extend ventrolaterally to become closely applied to the epibranchial placodes.
The cranial neural crests are more organized initially than the spinal neural crests.
Experimental studies (Johnston MC: A radioautographic study of the migration and fate of cranial neural crest cells in the chick embryo. Anat Rec 156:143–156, 1966) indicate that cranial neural crest cells in birds are precursors of a) sensory ganglion cells of cranial nerves, b) autonomic (motor) ganglion cells on cranial nerves, c) neurolemmal sheaths of cranial nerves and d) branchial arch cartilages.
One neural crest is associated with each of the four branchial arches:
First branchial arch → trigeminal crest → cranial nerve V
Second branchial arch (including otocyst) → facioacoustic crest → cranial nerves VII and VIII
Third branchial arch → glossopharyngeal crest → cranial nerve IX
Fourth branchial arch → vagal crest → cranial nerve X
Sensory preganglia begin to form within each crest near the placode and will become ganglia on the cranial nerves.
Trigeminal crest → trigeminal ganglion (V)
Facioacoustic crest → geniculate (VII), cochlear (spiral) and vestibular (VIII) ganglia
Glossopharyngeal crest → superior (jugular) and inferior (petrous) ganglia (IX)
Vagal crest → superior (jugular) and inferior (nodose) ganglia (X)
Accessory Nerve (XI)
The accessory nerve is subdivided into cranial and spinal parts. The cranial part is a caudal extension of the vagal crest and is considered a part of the vagus nerve.
The spinal part first appears as a longitudinal collection of cells along the dorsolateral surface of the rhombencephalon and upper spinal cord. Rostrally it joins with the cranial part before it proceeds along the dorsal side of the precardinal vein. It disappears in a mesodermal condensation just caudal to the cervical sinus where the sternocleidomastoid-trapezius premuscle mass will form.
Hypoglossal Nerve (XII)
The hypoglossal nerve begins as a short, poorly defined, longitudinal collection of cells along the ventrolateral surface of the rhombencephalon. This cell collection is closely related to a condensation of occipital myoblasts.
SPINAL NERVES
Spinal Neural Crests
The crest cells along the spinal part of the neural tube collect together in the form of a longitudinal band on either side of the tube. The band has a scalloped ventral border. It is broadest in the upper cervical region and tapers toward the tail region.
Collections of crest cells extend farthest ventrolaterally near the middle of each body segment. Such extensions occur in a regular manner and are most prominent in the upper cervical segments.
A spinal ganglion (sensory) begins to develop within each crest midway between the neural tube and the myotome.
In some segments, crest cells have begun to spread ventral to the neural tube around the dorsal aorta. They will form the ganglia of the autonomic nervous system and are motor to the viscera.
The upper limb bud forms at the level of the C-5 to T-1 neural crests. The lower limb bud forms at the level of the L-1 to S-2 neural crests.
Source: Atlas of Human Embryos.