DEVELOPMENT OF THE CEREBRAL HEMISPHERE
The layout of structures in the cerebral hemisphere is better understood by a knowledge of its development (Fig. 8.9).
The cerebrum is a derivative of the prosencephalon. We have seen that the prosencephalon is divisible into a mediandiencephalon and two lateral telencephalic vesicles (Fig. 6.11C). The telencephalic vesicles give origin, on either side, to the cerebral cortex and the corpus striatum. The diencephalon gives rise to the thalamus, hypothalamus and related structures. The telencephalic vesicles are at first small, but rapidly increase in size extending upwards, forwards and backwards. As a result of this enlargement, the telencephalon comes to completely cover the lateral surface of the diencephalon and eventually fuses with it. Thus, the cerebral cortex and corpus striatum come to lie lateral to the thalamus and hypothalamus.
With further upward, forward and backward extension of the telencephalic vesicles, the vesicles of the two sides come into apposition with each other above, in front of, and behind the diencephalon.
The cavity of the diencephalon forms the third ventricle, while the cavities of the two telencephalic vesicles form the lateral ventricles (Fig. 6.13).
Each lateral ventricle is at first a spherical space within the telencephalic vesicle. With the forward and backward growth of the vesicle, the ventricle becomes elongated anteroposteriorly. The posterior end of the telencephalic vesicle now grows downwards and forwards, to form the temporal lobe, and the cavity within it becomes the inferior horn. The ventricle thus becomes C-shaped. Finally, as a result of backward growth, the occipital pole of the hemisphere becomes established, the part of the ventricle within it becoming the posterior horn.