The fore brain, the organ of these functions, only attains this high level of development in the more advanced Placentals, and thus we have the simple explanation of the intellectual superiority of the higher mammals. The soul of most of the lower Placentals is not much above that of the reptiles, but among the higher Placentals we find an uninterrupted gradation of mental power up to the apes and man. In harmony with this we find an astonishing variation in the degree of development of their fore brain, not only qualitatively, but also quantitatively. The mass and weight of the brain are much greater in modern mammals, and the differentiation of its various parts more important, than in their extinct Tertiary ancestors. This can be shown paleontologically in any particular order. The brains of the living ungulates are (relatively to the size of the body) four to six times (in the highest groups even eight times) as large as those of their earlier Tertiary ancestors, the well-preserved skulls of which enable us to determine the size and weight of the brain. (FIGURE 2.303. Brain of a human embryo, twenty-four weeks old, halved in the median plane: right hemisphere seen from inside. (From Mihalkovics, natural size.) 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. In the lower mammals the surface of the cerebral hemispheres is quite smooth and level, as in the rabbit (Figure 2.304). 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 (Figures 2.292 and 2.293). 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.