The neural tube differentiates into brain vesicles ("-cephalon")
The earliest subdivisions of the brain have developed by the end of 4 weeks.
3 Primary Brain Vesicles
There are 3 primary brain vesicles:
Prosencephalon
Mesencephalon
Rhombencephalon
5 Secondary Brain Vesicles at 5 Weeks
At 5 weeks, they further develop into the secondary brain vesicles, and there are 5 of them:
The prosencephalon partitions into the telencephalon and diencephalon.
The mesencephalon remains as the mesencephalon.
The rhombencephalon partitions into the metencephalon and myelencephalon.
3 Flexures
There are 3 flexures:
Cephalic flexure (aka midbrain flexure)
Pontine flexure
Cervical flexure
Primary brain vesicles at 4 weeks
Primary Brain Vesicles
From cranial to caudal:
Prosencephalon
Mesencephalon
Rhombencephalon
Points of interest
The caudal extension of the neural tube later becomes the spinal cord.
The neural tube runs length of embryo until month 3 when spine growth starts to outpace spinal cord growth.
Secondary brain vesicles at 5 weeks
Secondary Brain Vesicles
From cranial to caudal:
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Specifics of development
The prosencephalon partitions into the telencephalon and diencephalon.
The mesencephalon remains as the mesencephalon.
The rhombencephalon partitions into the metencephalon and myelencephalon.
Points of interest
The pontine flexure within the metencephalon lays the groundwork for the floor of the fourth ventricle (ie, the rhomboid fossa, given its rhomboid shape). The opposite surface of the metencephalon connects as the 4th ventricle roof.
The optic vesicles lie within the diencephalon.
Each optic vesicle partitions into the optic cup and the optic stalk.
We can remember that the optic vesicle develops from the diencephalon because the optic pathway synapses in the lateral geniculate nuclei along the thalamus.
The optic cup then becomes the retina.
And the optic stalk becomes the optic nerve.
The Brain at 6 months
Derivations
The cerebrum is derived from the telencephalon.
The thalamic areas are derived from the diencephalon.
The midbrain is derived from the mesencephalon.
The pons and cerebellum are derived from the metencephalon.
The medulla oblongata is derived from the myelencephalon.
The spinal cord is derived from the neural tube.
Brain Development
The telencephalon blossoms into the large cerebral hemispheres.
The diencephalon is buried deep within the brain; it comprises the thalamic areas.
The brainstem comprises, from superior to inferior: the midbrain, pons, and medulla.
The cerebellum blossoms from the posterior pons: both are derived from the metencephalon.
The spinal cord extends from the bottom of the medulla oblongata.