The emergence of the telencephalic vesicles allows us to distinguish the final two major forebrain regions, the telencephalon and the diencephalon, from one another (have a look back at Figs. 8.1 and 8.2). The telencephalic vesicles give rise to the two cerebral hemispheres while the diencephalon remains as the rostral end of the neural tube. In the diencephalon, the lumen of the neural tube forms the tall narrow slit of the third ventricle (see Fig. 9.3). The roof of the third ventricle remains as a thin choroid membrane, which eventually invaginates to form a modest choroid plexus. The sulcus limitans persists in both walls of the third ventricle, where it is called the hypothalamic sulcus and delimits the thalamus above from the hypothalamus below. The hypothalamus grows more modestly than the thalamus during development, and each differentiates into several nuclear groups. Neither, however, expands as exuberantly as the cerebral hemispheres. In fact, the hemispheres are by far the most productive parts of the developing CNS in terms of ultimate volume and morphological differentiation.
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