1. Development of the Nervous system

2. EARLY DEVELOPMENT
By the beginning of the 3rd week of development, three germ cell layers become established, ectoderm, mesoderm and endoderm.
During the middle of the 3rd week, the dorsal midline ectoderm undergoes thickening to form the neural plate.
The margins of the plate become elevated, forming neural folds.
A longitudinal, midline depression, called the neural groove is formed.
The 2 neural folds then fuse together, to form the neural tube.
Formation of the neural tube is completed by the middle of the fourth week of development.

4. Neural tube is open at both ends, communicating freely with the amniotic cavity.
The cranial opening, the rostral neuropore closes at about 25th day & the caudal neuropore closes at about the 27th day

5. The cranial ⅓ of the neural tube represent the future brain
The caudal ⅔ represents the future spinal cord

6. Neural Tube Development Three-vesicles stage (End of 4th Week)
The cranial part of neural tube dilates to form brain vesicle. The caudal part remains narrow and cylindrical to give spinal cord
The brain vesicle grows and gives 3 dilatations named as:
Prosencephalon
Mesencephalon
Rhombencephalon

7. Neural Tube Development Five-vesicles stage (5th Week)

8. Brain Flexures By 4th week:
The neural tube grows rapidly and bends ventrally with the head fold, producing two flexures:
Midbrain (cephalic) flexure: In the region of midbrain.
Cervical flexure: between the hind brain & the spinal cord. HB MB FB HB
Later Pontine flexure appears in the hindbrain, in the opposite direction of the cephalic & cervical flexures, resulting in stretching and thinning of the roof of the hindbrain. 

9. Differentiation of Forebrain
The prosencephalon or the forebrain vesicle differentiates into:
a median part, the diencephalon, and
two lateral cerebral vesicles or (telencephalic vesicles.)
The lumen gives the 3rd ventricle and the 2 lateral ventricles.
Both cavities communicate with each other through a large interventricular foramen.(of Monro)

10. Development of the Cerebrum
The cerebrum develops from the Telencephalon

11. The cerebral hemispheres first appear on the day 32 as a pair of bubble-like outgrowths of the Telencephalon.
By 16 weeks, the rapidly growing hemispheres are oval in shape and cover the diencephalon.
The cerebral hemispheres expand in all directions.
Its medial wall becomes thin, flat and it is the site of choroid plexus of the lateral ventricle.

12. The wall of the telencephalon is formed of 3 layers:
Ependyma: lining the cavity of the lateral ventricle.
Mantle layer: Intermediate layer contains nerve cells (grey matter).
Marginal layer: Outer layer contains nerve fibers emerging from neuroblasts in the mantle layer (white matter).
As a result of myelination of nerve fibers, this layer takes on a white appearance and therefore is called the white matter of the spinal cord.

13. As development proceeds most of the nerve cells in the mantle layer migrate to the outer aspect of the marginal layer to form the cerebral cortex. The remaining cells form the basal ganglia (corpus striatum).

14. By the end of the 3rd month the surfaces of the cerebral hemispheres are smooth. By 4th month the grey matter grows faster than the white matter with the result that the cortex becomes folded into gyri separated by sulci.
The gyri and sulci effectively increase the surface area of the brain. The detailed pattern of gyri & sulci varies somewhat from individual to individual

15. Corpus striatum appears in 6th week in the floor of each cerebral hemisphere.
The cerebral cortex differentiates and the fibers passing to and from it, pass through the corpus striatum, dividing it into caudate nucleus & lentiform nucleus.
This fiber pathway forms the internal capsule.

16. Further expansion of cerebral hemisphere give C-shape to the hemisphere itself as well as the lateral ventricle.
Also the caudate nucleus elongates and assumes the shape of the lateral ventricle and remains related to it.

17. Development of the Cerebral Commissures
As the cerebral cortex develops, group of fibers, the commissures, connect the corresponding regions of the cortex in the two hemispheres.
These are:
Lamina terminalis.
Optic chiasma.
Anterior commissure.
Posterior commissure.
Hippocampal commissure.
Habenular commissure.
Corpus callosum. 6 5 4 7 1 3 2

18. The cortex covering the surface of the corpus striatum grows relatively slowly, so it is overgrown by the rest of the hemisphere and lies in the depth of the lateral sulcus. This is the insula.

19. Congenital anomalies of the brain Anencephaly
Due to failure of closure of the rostral neuropore of the neural tube…
Failure of complete development of the brain , skull and scalp

20. Microcephaly
The circumference of the head , including the brain is smaller than normal

21. Congenital hydrocephalus
Abnormal Accumulation of CSF inside the brain cavities