Three flexures form in the part of the embryonic neural tube that develops into the brain.

At four weeks gestational age in the human embryo, the neural tube has developed at the cranial end into three swellings – the primary brain vesicles.

The space into which the cranial part of the neural tube is developing is limited.

By the fourth week in the human embryo, at its cranial end, three swellings have formed as primary brain vesicles.

By the fifth week further flexion has taken place and the five secondary brain vesicles have formed.