By the end of the second month some reflex activity is demonstrated. Lightly stroking the lips causes contralateral flexion of the neck and upper trunk. Some of the sensory and motor neurons therefore have functional connections both centrally and peripherally.
GENERAL
The brain continues to enlarge at a faster rate than the spinal cord. This is especially evident in the cerebral hemispheres (vesicles). As the head raises, the rhombencephalon assumes a more vertical position and the cervical flexure becomes less acute. The cephalic and pontine flexures become more acute. The spinal cord begins to lag in its ability to keep pace with the increasing length of the vertebral column. As a result, the caudal end of the spinal cord begins to move cranially in relation to the vertebral column. It remains attached to the coccygeal region by the primitive filum terminale.
The ectomeninx differentiates into an outer layer that has osteogenic or chondrogenic properties and an inner layer called the dura mater. Venous sinuses or plexuses separate the two layers in the cranium. In the vertebral column the two layers are separated by a venous plexus around which a large epidural space will subsequently develop. The endomeninx differentiates into the pia mater around the blood vessels on the surface of the CNS and the arachnoid trabeculae that attach to the deep surface of the dura mater. The arachnoid appears in late fetal life as a delamination from the deep aspect of the dura mater. When this occurs, a very narrow subdural space separates the dura mater and arachnoid and a broad subarachnoid space containing cerebrospinal fluid separates the arachnoid from the pia mater.
BRAIN
Telencephalon
Cerebral Vesicle (Cerebral Hemisphere)
Striatal Part
The cerebral vesicle expands at a rapid rate in a semicircular manner, growing dorsally first, then caudally. The striatal ridge in the floor of the lateral ventricle at the interventricular foramen also grows in a similar manner. The rostral part of the ridge divides into medial and lateral portions. The large basal ganglia subsequently differentiate from the striatal part of the cerebral vesicle.
The primordial piriform cortex at the surface is close to the primordial olfactory bulb where the olfactory nerves enter.
Suprastriatal Part
The lateral ventricle follows the dorsal, then caudal expansion of the cerebral vesicle and thereby produces an inferior horn. Likewise, the choroid fissure and plexus in the medial wall at the interventricular foramen are pulled in a similar manner.
The primordial hippocampus dorsal to the choroid fissure follows the same growth pattern lying first in the medial wall of the inferior horn, then in the floor. The amygdala area lies in the roof of the inferior horn between the rostral end of the horn and the basal ganglia. The wall of the telencephalon, in the area of the primordial olfactory bulb, evaginates to form the olfactory bulb. The ventricular cavity extends initially into the bulb but later disappears.
A thick layer of neuroblasts develops in the primordial neopallial cortex forming the superficial cortical layer of the neopallial cortex. Frontal, parietal, occipital and temporal lobe areas become apparent. Growth of the region between the frontal and temporal areas becomes depressed forming the insular area that lies lateral to the striatal part of the cerebral vesicle.
Telencephalon Medium
The preoptic recess is a distinct, isolated portion of the third ventricle and is bound on each side by the preoptic area. The preoptic area is separated from the hypothalamus by the optic groove and is delimited dorsally by the anterior commissure area where olfactory fibers cross the midline.
Diencephalon
Epithalamus
The epithalamus develops from the roof plate and the adjacent, dorsal part of the lateral wall. Its growth does not keep pace with the dorsal thalamus and it is reduced to a small region above the third ventricle.
The pineal bud becomes a distinct evagination of the roof plate and subsequently becomes the pineal gland (epiphysis).
The habenular area where the habenular nuclei develop is rostral to the pineal gland. The habenular commissure area appears as a clear area in the marginal layer in the midline. The posterior commissure area is similar in appearance caudal to the pineal gland.
Dorsal Thalamus
The dorsal thalamus enlarges greatly and makes up most of the dorsal part of the diencephalon. It lies dorsal to the hypothalamic sulcus, the formation of which is described below.
The internal capsule area is lateral. Caudally it is continuous with the mesencephalon.
Ventral Thalamus
As the dorsal thalamus expands, the ventral thalamus moves from a position near the midline to one lateral to the hypothalamus.
As this occurs, the sulci medius and ventralis blend to form the hypothalamic sulcus, which continues rostrally into the preoptic recess as the optic groove.
Hypothalamus
The hypothalamus lies in the most ventral part of the diencephalic wall below the hypothalamic sulcus. Its rostral boundary is the optic chiasma; its caudal boundary is the mammillary area where the small, spherical mammillary body will develop on each side of the midline. The hypothalamus is continuous rostrally with the preoptic area of the telencephalon. Caudally it is adjacent to the tegmentum of the mesencephalon. The neuroblasts in the wall will give rise to the hypothalamic nuclei.
The neurohypophyseal bud gives rise to the neurohypophysis that is located in the midline just caudal to the optic chiasma. The tuberal part of the adenohypophysis covers it laterally.
Mesencephalon
Tectum
A longitudinal elevation called the collicular ridge develops in the roof plate. A transverse groove subsequently divides the ridge into superior and inferior colliculi that are relay centers of the visual and auditory senses, respectively.
The thick mantle layer gives rise to neuroblasts that migrate into the marginal layer in the form of sheets. This gives the tectum its stratified appearance.
Tegmentum
The neuroblasts in the tegmentum will differentiate into the motor nuclei of cranial nerves III and IV, the red nucleus and the substantia nigra. Those that remain dispersed contribute to the reticular formation.
The alar plate possibly contributes some neuroblasts to the tegmentum.
Basis Pedunculi Area
The basis pedunculi area located ventral to the tegmentum will be composed of fibers that originate primarily from the neopallial cortex.
It is continuous rostrally with the internal capsule area and caudally with the pons area.
Metencephalon
Pons Area
Basal Portion
Many of the fibers from the neopallial cortex will pass through the marginal layer making up the pyramidal tract area.
Some of these fibers will relay in the pons area and course to the cerebellum.
Tegmentum
The alar and basal plates are separated by a shallow sulcus limitans.
Neuroblasts thought to be from the alar plate of the myelencephalon extend rostrally into the pons area where they form the primordial superior olivary nucleus and, at a later time, the pontine nuclei.
Neuroblasts in the mantle layer of both the metencephalon and myelencephalon begin to segregate themselves into groups that become the nuclei of several cranial nerves. The motor nuclei are derived from the basal plate and the sensory nuclei are derived from the alar plate.
Motor nuclei—Three different columns of motor nuclei develop in the basal plate. Somatic motor nuclei differentiate medially near the midline; branchiomotor (special visceral motor) nuclei develop in the intermediate region, and general visceral motor nuclei form in the lateral part of the basal plate. The primordial abducens (somatic motor) and facial (branchiomotor) nuclei are among the first motor nuclei in the metencephalon to become apparent.
Sensory nuclei—Three different columns of sensory nuclei develop in the alar plate, their relative positions being a mirror image of the motor nuclei. General visceral sensory nuclei differentiate medially next to the sulcus limitans, special visceral sensory nuclei develop in the intermediate region and general somatic sensory nuclei form laterally next to the developing cerebellum. A fourth column called special somatic sensory nuclei is described in the adult. These nuclei lie between the special visceral and general somatic sensory nuclei and are relay centers for the vestibulocochlear nerve. The primordial nucleus of the spinal tract of V (general somatic sensory) and the rostral part of the primordial vestibular nucleus (special somatic sensory) are among the first sensory nuclei in the metencephalon to differentiate. The clear region lateral to the primordial nucleus of the spinal tract of V represents the spinal tract of V area.
Cerebellum
The intraventricular portion of the rhombic lip initially enlarges at a more rapid rate than the extraventricular portion and bulges into the fourth ventricle. The extraventricular will subsequently become larger than the intraventricular portion. Together they will form the cerebellar hemispheres.
In the rostral part of the metencephalon the rhombic lips will join together in the midline where the cerebellar vermis develops.
Medulla Oblongata (Myelencephalon)
All of the motor and sensory nuclear columns of the metencephalon are continuous into the medulla oblongata and retain their same relative positions. The roof plate narrows in the caudal part of the medulla oblongata bringing the alar plate and the related sensory nuclei into a dorsolateral position.
Basal Plate
The primordial hypoglossal nucleus (general somatic motor) becomes evident in the floor of the fourth ventricle next to the midline.
Alar Plate
A large part of the primordial vestibular nucleus develops in the special somatic sensory nuclear column in the rostral portion of the medulla oblongata. Several primordial nuclei become apparent in the general somatic sensory nuclear column in the caudal portion of the medulla oblongata. From medial to lateral they are the primordial nucleus gracilis, nucleus cuneatus and nucleus of the spinal tract of V. A large number of neuroblasts thought to be derived from the alar plate collect in the ventral portion of the medulla oblongata to form the primordial inferior olivary and arcuate nuclei, both of which will send fibers to the cerebellum.
Tract Areas
Several major tracts become evident as clear, less cellular areas. The pyramidal tract area is well defined in the ventral portion of the medulla oblongata. It will be composed of fibers from the neopallial cortex that descend to the motor nuclei in the spinal cord. Next to the midline the medial lemniscus area becomes evident and will contain primarily ascending fibers from the sensory nuclei in the spinal cord on their way to the thalamus. Many of the fibers cross to the opposite side in the sensory decussation area. The spinal tract of V area will contain descending sensory fibers from the skin of the face and auricle. It begins in the pons area just lateral to its primordial nucleus and will extend through the medulla oblongata. It will be continuous with a similar region in the dorsolateral part of the spinal cord (Lissauer’s tract).
Fourth Ventricle
The diamond shaped fourth ventricle narrows in the caudal portion of the medulla oblongata and becomes continuous with the neural (central) canal of the spinal cord.
The roof fuses with the vascular layer of the endomeninx (pia mater) thereby forming the tela choroidea. Folds of the tela project into the ventricle as the choroid plexus where cerebrospinal fluid will be produced. Cerebrospinal fluid will circulate throughout the brain ventricles. A midline outpocketing of the roof called the ependymal diverticulum communicates with the fourth ventricle through the median aperture. The diverticulum subsequently disappears after which the cerebrospinal fluid can pass from the ventricle into the subarachnoid space. The median aperture is then called the foramen of Magendie. A similar aperture called the foramen of Luschka will also appear in the roof on each side of the midline.
When the cerebellum enlarges it pushes the roof caudally and ventrally. Eventually the roof lies below the caudal part of the cerebellum.
SPINAL CORD
As the layers of the spinal cord expand, the neural canal is reduced to a tiny channel lined with ependyma, called the central canal. Its dorsal walls join in the midline to form the dorsal median septum. The central canal dilates caudally at the tip of the spinal cord into the terminal ventricle.
The neuroblasts are located primarily in the mantle layer, which gives rise to the grey matter. The marginal layer, containing predominantly nerve fibers, forms the white matter at the periphery of the grey matter. The grey matter arranges itself in the shape of a butterfly on cross section with dorsal and ventral horns or columns that arise from the alar and basal plates, respectively. A lateral horn develops in the thoracic and upper lumbar segments and is the seat of visceral motor nuclei. Sensory nuclei reside in the dorsal horn, somatic motor nuclei are located in the ventral horn. The dorsal and ventral roots of the spinal nerves divide the white matter into dorsal, lateral and ventral funiculi.
The ventral median sulcus on the brain is continuous onto the spinal cord with a deeper groove called the ventral median fissure. The ventral (anterior) spinal artery travels in the fissure. A shallower dorsal median sulcus appears in the midline on the dorsal surface.
PERIPHERAL NERVOUS SYSTEM
Because the vertebral column lengthens at a faster pace than the spinal cord, the spinal cord recedes cranially in the vertebral canal. In order to accommodate this differential growth pattern the spinal nerve roots attached to the caudal part of the cord must increase in length. As the tip of the spinal cord recedes progressively higher in the vertebral canal, the primitive filum terminale as well as progressively more spinal nerve roots increase in length. At birth the tip of the spinal cord is located at approximately the L-4 vertebra; in the adult it is located at approximately the L-2 vertebra. The many spinal nerve roots in the vertebral canal between the end of the spinal cord and the coccyx are collectively called the cauda equina (horse’s tail).
As the vertebral column develops, all but the most caudal spinal ganglia lie in the intervertebral foramina. Since the sacral foramina develop peripheral to the sacral spinal ganglia, these ganglia are located in the caudal part of the vertebral canal. In the adult the C-1 spinal ganglion is usually not apparent. The C-1 spinal nerve is predominantly motor.
All of the named peripheral nerves can be identified by the end of the eighth week. As viscera and muscle masses shift in their position, their nerves accommodate the shifts by increasing in length. The heart and diaphragm receive most of their innervation when they are in the cervical region. Since their nerves (superior, middle and inferior cervical cardiac nerves of the heart and phrenic nerve of the diaphragm) follow them as they migrate caudally, they begin in the neck and descend through the root of the neck and thorax. Likewise, the larynx receives its motor innervation from the vagus when the aortic arches are farther cranially. When the arches migrate caudally with the heart, the recurrent laryngeal nerve follows causing it to recur in its course to the larynx. In the adult it passes around the aortic arch and ligamentum arteriosum on the left, which were derived from the fourth and sixth arches, respectively. Since on the right side the distal part of the sixth arch disappears, the nerve loops around the proximal part of the subclavian artery that is derived from the fourth arch.
SUPRARENAL GLAND
The suprarenal gland is initially composed primarily of cortex with only a scant amount of medulla medially. The very large cortical mass lies on the dorsomedial surface of the definitive kidney. During late fetal life the medulla will be overgrown by the cortex causing it to occupy a more central position within the cortex. The cortex formed during the embryonic period becomes the fetal cortex, which disappears after birth. It is replaced later by the definitive cortex that is derived from smaller cells at the periphery.
Source: Atlas of Human Embryos.