5 Vesicle Stage, Week 5: By the end of the 4th week the 3 part brain begins to develop 5 vesicles. The forebrain (prosencephalon) gives rise to, a) the paired lateral telencephalic vesicles which bud off from the prosencephalon and will become the cerebral hemispheres and b) the diencephalon (from which the optic vesicles extend). At the cephalic flexure, the mesencephalon remains tubular and undivided. The hindbrain (rhombencephalon) subdivides into a metencephalon and a more caudal myelencephalon.

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