By the cranial flexure the brain is bent at the junction of the first and second cerebral vesicles;
Fig. 289. Chick on the third day, seen from beneath as a transparent object, the head being turned to one side. (Foster and Balfotir).
a'. False amnion, a. Amnion. CH. Cerebral hemisphere. FB., MB., HB , Anterior Middle and Posterior cerebral vesicles. op. Optic vesicle. ot. Auditory vesicle, ofv. Omphalo-mesenteric veins. Ht- Heart. Ao. Bulbus arteriosus. Ch. Notochord. Of.a. Omphalo-mesenteric arteries. Pv. Proto-vertebrae. x. Point of divergence of the splanchno-pleural folds, y. Termination of the foregut, V.
the first is thus turned downward, leaving the second as the most anterior part of the brain.
The upper walls of the middle cerebral vesicle are developed into the corpora quadrigemina.
The cavity of this vesicle persists as a narrow channel, forming a communication between the third ventricle in front and the fourth ventricle behind, and receives the name in the adult brain of the iter a tertio ad quartum ventriculum. The crura cerebri arise from the lower wall of this middle vesicle.