3 Vesicle Stage, Week 4: The complex shape of the adult brain is determined during embryogenesis by flexures that form early in development. These bends are due to tremendous cell proliferation, differential growth, and because the brain develops in the confined space of the cranial vault.

The anterior neuropore closes at about the 24th day and the posterior neuropore closes on the 26th day. During the 4th week the expanded cephalic portion of the neural tube, which forms the brain, subdivides into 3 primary vesicles, forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The optic vesicle of the forebrain appears early, even before closure of the anterior neuropore. The caudal portion of the neural tube, which forms the spinal cord, remains essentially a simple tube.

By about the end of the 3rd week this 3 part brain begins to assume a “C”-shape by the formation of cephalic flexure at the level of the mesencephalon; at the end of the 4th week a cervical flexure develops between the hindbrain and spinal cord.
 

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