GENERAL
Since the entire brain enlarges at a faster rate than the spinal cord, the latter becomes relatively smaller and shorter.
The primitive meninx differentiates into an outer, condensed layer called the ectomeninx and an inner, loosely arranged layer called the endomeninx located adjacent to the surface of the brain and spinal cord.
BRAIN
Telencephalon
Cerebral Vesicle
Striatal Part
The striatal part enlarges considerably and bulges into the third ventricle at the level of the interventricular foramen where it produces the striatal ridge.
The primordial piriform cortex (paleopallium) differentiates at the surface of the striatal part and will receive secondary olfactory fibers.
Suprastriatal Part
The suprastriatal part expands rostrally, dorsally and caudally producing the pallium (primordial cerebral cortex). The caudal portion begins to cover over the diencephalon.
The layer of tissue in the midline that separates the pallium on each side will become the falx cerebri. The medial wall of the pallium thins and invaginates into the lateral ventricle to form the tela choroidea. The line of imagination, called the choroid fissure, first appears at the level of the interventricular foramen.
Dorsal to the choroid fissure the medial wall bulges into the lateral ventricle as the primordial hippocampus. The clear outer layer represents the primordial hippocampal cortex (arch/pallium). It can be subdivided into a ventral area, the primordial gyrus dentatus, and a more dorsal area, the primordial cornu Ammonis. The medial surface in front of the lamina terminalis is the precommissural septum (parolfactory area). A small prominence on the ventromedial surface represents the primordial olfactory bulb, which receives the olfactory nerve fascicles. Caudal to the bulb is the primordial tuberculum olfactorium.
Most of the pallium lies between the primordial hippocampal cortex and primordial piriform cortex. This large area, referred to as the primordial neopallial cortex, is associated with non-olfactory sensations and will form many connections with the thalamus.
Telencephalon Medium
The preoptic area and recess are seen rostral to the optic chiasma of the diencephalon in the floor of the third ventricle.
The lamina terminalis in front of the recess represents the rostral end of the brain. It gradually diminishes in relative size and is covered by the pallium.
The paraphysis forms as an evagination of the paraphyseal arch in the roof of the third ventricle near the tel- and diencephalon junction. Usually it disappears during early fetal development (100 mm).
Diencephalon
A third sulcus, the sulcus medius, appears in the lateral wall between the sulcus dorsalis and sulcus ventralis. The three sulci divide the wall into four areas. From dorsal to ventral they are the epithalamus, dorsal thalamus, ventral thalamus and hypothalamus. A lightly staining, less cellular area called the zona limitans intrathalamica becomes evident near the sulcus medius and helps to separate the dorsal thalamus from the ventral thalamus. Increased proliferation in the mantle layer causes the hypothalamus to bulge into the third ventricle.
The optic chiasma becomes a thick band in the rostral part of the floor plate that connects the optic stalks across the midline. Caudal to the chiasma the solid neurohyphyseal bud is flanked on each side by the tuberal part of the adenohypophysis. The mamillary area remains as a slight outward bulge in the caudal part of the floor plate.
No significant changes occur in the pineal bud area in the caudal part of the roof plate.
Mesencephalon
Lamination begins to appear in the mantle layer of the tectum.
The tegmentum and basis pedunculi area subdivisions increase in size. A midline depression called the inter-peduncular fossa forms between the peduncular area on each side in the region of the cephalic flexure.
As the mesocoele narrows it becomes the cerebral aqueduct connecting the third and fourth ventricles.
Cranial nerve III attaches to the wall of the interpeduncular fossa. Cranial nerve IV attaches to the roof of the isthmus.
Rhombencephalon
General
The pontine flexure becomes more acute.
The thin, diamond shaped roof of the fourth ventricle continues to enlarge with the cranial part forming the roof of the metencephalon and the caudal part forming the roof of the myelencephalon.
There is no longer any evidence of the rhombomeres.
A midline raphe of crisscrossing fibers becomes prominent in the floor plate region along the entire length of the rhombencephalon.
Metencephalon
Pons Area
As the marginal layer of the basal plate thickens around the attachment of the trigeminal nerve it becomes the basal portion of the pons. The ventral median sulcus becomes apparent on the ventral surface.
The remainder of the basal plate together with the medial segment of the alar plate constitute the tegmentum of the pons. Neuroblasts in the mantle layer of the basal plate will form the motor nuclei of cranial nerves V, VI and VII. Neuroblasts in the alar plate will contribute to the formation of the sensory nuclei of cranial nerves V, VII, VIII and X.
Cranial nerve V attaches to the metencephalon in the middle of the pons area. Cranial nerves VI, VII and VIII attach at the caudal edge of the pons area.
Rhombic Lip (Cerebellum)
The rhombic lip is a slight thickening of the lateral segment of the alar plate where the roof of the fourth ventricle attaches. It can be divided into intraventricular and extraventricular portions.
Myelencephalon
The myelencephalon becomes a distinct subdivision of the adult brain between the pons area and the C-1 spinal nerve called the medulla oblongata.
Neuroblasts in the basal plate will form the motor nuclei of cranial nerves IX, X, XI (cranial part) and XII. Neuroblasts in the alar plate will contribute to the formation of the sensory nuclei of cranial nerves V, VII, VIII, IX and X.
The pyramidal tract area thickens on each side of the midline raphe.
The rootlets of cranial nerve XII attach just lateral to the pyramidal tract area. Cranial nerves IX, X and XI attach in series to the lateral wall of the myelencephalon.
SPINAL CORD
The spinal cord does not keep pace with the enlargement of the brain and becomes relatively narrower and shorter.
The layers increase in thickness converting the neural canal into a vertical slit that is slightly expanded at the sulcus limitans.
Caudal to the coccygeal spinal ganglion the spinal cord narrows and ends abruptly. It continues into the tail bud remnant as the primitive filum terminale. The primitive filum terminale is initially very short and contains a tiny canal that is continuous with the neural canal.
PERIPHERAL NERVOUS SYSTEM
GENERAL
The diameter of the cranial and spinal nerves together with their sensory ganglia is relatively smaller since the nerves can not keep pace with the enlargement of the CMS.
Many named branches become apparent as the nerves course into the peripheral areas.
CRANIAL NERVES
Olfactory nerve (I)—The olfactory nerve passes as a group of fascicles from the olfactory epithelium to the primordial olfactory bulb. The vomeronasal nerve and nervus terminalis course from the vomeronasal organ and front part of the nasal septum, respectively, to the medial side of the primordial bulb.
Optic nerve (II)—The optic stalk lumen is barely visible because of the increased quantity of fibers from the optic cup causing the lumen to narrow. After the lumen disappears, the optic stalk is called the optic nerve.
Oculomotor (III), trochlear (IV) and abducens (VI) nerves— These nerves terminate in distinct premuscle masses surrounding the optic cup.
Trigeminal nerve (V)—The three divisions or branches of the trigeminal nerve course into their particular area of the face. The infraorbital branch of the maxillary nerve and the inferior alveolar and lingual branches of the mandibular nerve become apparent.
Facial nerve (VII)—The facial nerve does not yet spread onto the face but instead terminates in the cervicomandibular area. More proximally the chorda tympani, greater petrosal, posterior auricular and digastric branches can be identified. The chorda tympani branch joins the lingual nerve.
Vestibulocochlear nerve (VIII)—Short branches from the vestibular ganglion course to the future receptor area of the semicircular canals, saccule and utricle. The cochlear (spiral) ganglion gives branches to the cochlear pouch and spirals along with it.
Glossopharyngeal nerve (IX)—Before it terminates in the surface on the caudal part of the tongue, the glossopharyngeal nerve gives a minute branch to the stylopharyngeus premuscle mass.
Vagus nerve (X)—The vagus nerve joins its counterpart on the other side around the esophagus to form the esophageal plexus, which passes into the upper abdominal viscera as anterior and posterior vagal trunks.
Spinal accessory nerve (XI)—As the sternocleidomastoid and trapezius muscles become separate masses, they each receive a separate branch from the spinal accessory nerve.
Hypoglossal nerve (XII)—Separate branches of the hypoglossal nerve course to the muscles of the tongue as each muscle differentiates.
SPINAL NERVES
The branches of the nerve plexuses can be identified.
AUTONOMIC NERVOUS SYSTEM
Neuroblasts giving rise to postganglionic neurons collect together to form identifiable ganglia near their definitive position.
SYMPATHETIC NERVOUS SYSTEM
The individual ganglia of the sympathetic trunk become apparent. The superior, middle and inferior cervical ganglia are especially prominent because of their large size.
Many sympathetic neuroblasts collect together in the vicinity of the celiac, superior mesenteric and inferior mesenteric arteries to form preaortic ganglia with the same names. These ganglia are connected to the lower thoracic segments of the spinal cord by the splanchnic nerves and rami communicans.
Suprarenal gland—The cortex is a large acidophilic mass between the dorsal aorta and the cranial end of the mesonephros and gonad. The medulla is dorsomedial to the cortex and is composed of prechromaffin cells.
PARASYMPATHETIC NERVOUS SYSTEM
Parasympathetic neuroblasts collect in the head region to form four ganglia in close association with the divisions of the trigeminal nerve. Their origin is not clear as they are influenced by cranial nerves other than the trigeminal. The ganglia are as follows:
The ciliary ganglion lies close to the ophthalmic division near the optic cup but receives its messages from the brain by the oculomotor nerve.
The pterygopalatine ganglion is located on the maxillary division but receives brain messages by way of the facial nerve.
The submandibular ganglion is close to the mandibular division but is also connected to the brain by way of the facial nerve.
The otic ganglion is located on the mandibular division but receives impulses from the brain through the glossopharyngeal nerve.
The parasympathetic neuroblasts associated with the general visceral branches of the vagus nerve are scattered throughout the thoracic and upper abdominal viscera. Their origin is unclear but they may arise from crest cells in the rhombencephalic region. Since these neuroblasts never collect in large numbers, ganglia are not described.
Neuroblasts collect in the vicinity of the pelvic viscera and receive messages from the sacral part of the spinal cord by way of the pelvic splanchnic nerves. They also have an unclear origin but likely are derived from the neural crests in the sacral region.
Source: Atlas of Human Embryos.