It is particularly interesting to glance at the central nervous system of the vertebrates, the great stem of which we regard ourselves as the crowning point. Here again the anatomical and embryological facts speak a clear and unambiguous language. In all vertebrates, from the lowest fishes up to man, the psychic organ makes its appearance in the embryo in the same form--a simple cylindrical tube on the dorsal side of the embryonic body, in the middle line. The anterior section of this "medullary tube" expands into a club-shaped vesicle, which is the beginning of the brain; the posterior and thinner section becomes the spinal cord. The cerebral vesicle divides, by transverse constrictions, into three, then four, and eventually five vesicles. The most important of these is the first, the _cerebrum_, the organ of the highest psychic functions. The more the intelligence develops in the higher vertebrates, the larger, more voluminous, and more specialised does the cerebrum become. In particular, the grey mantle or cortex of the cerebrum, its most important part, only attains in the higher mammals the degree of quantitative and qualitative development that qualifies it to be the "organ of mind" in the narrower sense. Through the famous discoveries of Paul Flechsig eleven years ago we were enabled to distinguish eight fields in the cortex, four of which serve as the internal centres of sense-perception, and the four that lie between these are the thought-centres (or association-centres) of the higher psychic faculties--the association of impressions, the formation of ideas and concepts, induction and deduction. This real organ of mind, the _phronema_, is not yet developed in the lower mammals. It is only gradually built up in the more advanced, exactly in proportion as their intelligence increases. It is only in the most intelligent forms of the placentals, the higher ungulates (horse, elephant), the carnivores (fox, dog), and especially the primates, that the phronema attains the high grade of development that leads us from the anthropoid apes direct to the savage, and from him to civilised man.