The neural tube cranial to the fourth pair of the somites develops into the brain. During the 4th week, three primary brain vesicles are formed: the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon). Two flexures appear simultaneously: the cervical flexure at the junction of the hindbrain and the spinal cord, and cephalic flexure in the midbrain region.
During the 5th week, the forebrain divides into two vesicles, the telencephalon (future cerebral hemispheres) and diencephalon. The hindbrain partly divides into the metencephalon (future pons and cerebellum) and myelencephalon, separated by the pontine flexure.
The cavity of the rhombencephalon is known as the fourth ventricle, that of the diencephalon as the third ventricle, and those of the cerebral hemispheres as the lateral ventricles.

1. Prosencephalon
2. Mesencephalon
3. Rhombencephalon

1. Telencephalon
2. Lateral ventricle
3. Interventricular foramen of Monro
4. 3rd ventricule
5. Optic cup
6. 4th ventricle
7. Future aqueduct of Sylvius
8. Central canal

1. Diencephalon
2. Optic cup
3. Telencephalon
4. Primitive cerebral hemisphere
5. Metencephalon
6. Mesencephalon
7. Rhombencephalic isthmus
8. Myelencephalon
9. Pontine flexure

1. Diencephalon
2. Mesencephalon
3. Myelencephalon
4. Telencephalon
5. Metencephalon
6. Rhombencephalic isthmus
7. Roof of rhombencephalon
8. Central canal

1. Rhombencephalon (hindbrain)
2. Diencephalon
3. Heart
4. Prosencephalon (forebrain)
5. Optic outgrowth
6. Cervical flexure
7. Midbrain flexure
8. Myelencephalon
9. Mesencephalon (midbrain)
10. Metencephalon
11. Telencephalon
12. Spinal cord
13. 4th ventricle
14. Pontine flexure