Cervical flexure: The cervical flexure occurs at the junction of the metencephalon and spinal cord.Mesencephalic flexure: The mesencephalic flexure occurs around the mesencephalon.From Larsen (12).Д¬oncave bending of the neuraxis (primary flexures) occurs during the fourth embryonic week. Myelencephalon: The myelencephalon develops into the medulla.ДЇormation of the secondary vesicles: Shown are the five secondary vesicles that form during the fifth embryonic week.Metencephalon: The metencephalon forms the pons and cerebellum.Mesencephalon: The mesencephalon remains unchanged and gives rise to the tectum (superior and inferior colliculi), midbrain tegmentum, and cerebral peduncles.Diencephalon: The diencephalon develops into the hypothalamus, thalamus, epithalamus, neural retina, and pineal body.Telencephalon: The telencephalon forms the cerebrum and basal ganglia.From Larsen (12).Дuring the fifth embryonic week, five secondary vesicles develop from the primary vesicles: Prosencephalon вЂ“ two secondary vesicles

The cranial neural tube during the fourth embryonic week divides into three morphologically and histochemically distinct primary vesicles:ДЇormation of the primary vesicles: Formation and progression of the mesencephalic and cervical flexures are shown. The spinal cord is caudal to the myelencephalon and remains unsegmented. We conclude that differential growth may be necessary for mesencephalic flexure in the chick embryo, but other factors that decide the disposition of the products of growth in space must determine the shape.The formation of the cranial vesicles and flexures provides further development and segmentation of the brain and occurs in two phases ( 3, 10, 11, 12). The mesencephalon of explanted heads bends, but the prosencephalon fails to elongate. We reduced cerebrospinal fluid pressure during flexure by explanting heads to the chorioallantoic membrane, or into New cultures. We deleted brain parts adjacent to the mesencephalon before flexure and the mesencephalon bent, so migration of cells from or to these adjacent parts to contribute to the differential growth of the mesencephalon is unlikely. There are sufficient numbers of mitoses in the brain to account for the observed tissue growth, assuming accompanying cell enlargement. However, prosencephalon, which does not bend, also has differential growth between roof and floor, but the growth is taken up in complex changes of shape other than flexure. Both roof and floor increase in thickness, and mesencephalic roof volume increases twice as much as floor volume. As expected, during flexure the mesencephalic roof elongates much more than the floor.

We measured, during these stages, the lengths, thicknesses, and volumes of the floor and roof of the mesencephalon and of the prosencephalon. We have measured the changing angle of flexure through time and find that flexure occurs between chick embryo stages 10 to 15.

Most sources say the cephalic flexure is caused by differential growth. The cephalic flexure, found in all vertebrate brains, is a ventrally directed bend through the mesencephalon, and a ventral bulging and elongation of the prosencephalon.

Cephalic flexture