Fig. 301—Brain of an ox-embryo, two inches in length. (From Mihalkovics.) Left view; the lateral wall of the left hemisphere has been removed, st corpora striata, ml Monro-foramen, ag arterial plexus, ah Ammon’s horn, mh middle brain, kh cerebellum, dv roof of the fourth ventricle, bb pons Varolii, na medulla oblongata.
Fig. 302—Brain of a human embryo, twelve weeks old. (From Mihalkovics.) Seen from behind and above. ms mantle-furrow, mh corpora quadrigemina (middle brain), vs anterior medullary ala, kh cerebellum, vv fourth ventricle, na medulla oblongata.
In the lower mammals the surface of the cerebral hemispheres is quite smooth and level, as in the rabbit (Fig. 304). Moreover, the fore brain remains so small that it does not cover the middle brain. At a stage higher the middle brain is covered, but the hind brain remains free. Finally, in the apes and man, the latter also is covered by the fore brain. We can trace a similar gradual development in the fissures and convolutions that are found on the surface of the cerebrum of the higher mammals (Figs. 292, 293). If we compare different groups of mammals in regard to these fissures and convolutions, we find that their development proceeds step by step with the advance of mental life.
Fig. 303—Brain of a human embryo, twenty-four weeks old, halved in the median plane: right hemisphere seen from inside. (From Mihalkovics.) rn olfactory nerve, tr funnel of the intermediate brain, vc anterior commissure, ml Monro-foramen, gw fornix, ds transparent sheath, bl corpus callosum, br fissure at its border, hs occipital fissure, zh cuneus, sf occipital transverse fissure, zb pineal gland, mh corpora quadrigemina, kh cerebellum.