Formation of the secondary cerebral vesicles and the cerebral flexures
Over the course of the fifth week, a subdivision of the prosencephalons occurs into the telencephalon and the diencephalon. Parallel to this from the rhombencephalon the metencephalon and the myelencephalon arise.
Since the mesencephalon experiences no further articulation, the brain now consists of 5 secondary cerebral vesicles
The kinks in the region of the cranial flexure (flexura mesencephalica) and cervical or cranial flexure (flexura cervicalis) increase and between the metencephalon and the myelencephalon a bending of the neural tube in the other direction occurs, the pontine flexure (flexura pontina).
From the sixth week a distinct growth of the cerebral vesicles of the telencephalons on both sides occurs and the appearance of further bending leads to a stronger spreading of the derivates of the roof plate (dorsal collection in the visceral and somatosensory area) as opposed to the derivates of the floor plate (ventral collection in the visceral and somatomotoric area). At the same time, massive cell movements take place that lead to a collecting of gray substances in the periphery so that, centrally, white substances dominate at the end. This represents a distinct difference between the brain and spinal cord as will be shown in the context of their respective development. The tremendous development of the hemisphere vesicles of the telencephalon from the 6th and 7th weeks distinguishes humans from the other higher mammals
One notes that at around the eighth week (stage 23) the floor plate does not extend past the third cerebral vesicle. Thus motoric core regions are absent rostral of the mesencephalon. In contrast, the roof plate reaches the diencephalon.
At around the 38th day (stage 16) a ventrally directed rolling up of the neural tube around the cranial end of the notochord takes place. In this the third vesicle (metencephalon) gradually comes to lie on top while the advancing bending in the region of the cranial bend has the result that the most anterior two cerebral vesicles are put in front of the notochord.