In all Craniota, that is, in all vertebrate animals possessing skull and brain, the brain, which is at first only the bladder-shaped dilatation of the anterior end of the spinal marrow, divides into five bladders lying one behind the other, four superficial, transverse in-nippings being formed. These _five brain-bladders_, out of which afterwards arise all the different parts of the intricately constructed brain, can be seen in their original condition in the embryo represented in Fig. 7. It is just the same whether we examine the embryo of a dog, a fowl, a lizard, or any other higher vertebrate animal. For the embryos of the different skulled animals (at least the three higher classes of them, the reptiles, birds and mammals) cannot be in any way distinguished at the stage represented in Fig. 7. The whole form of the body is as yet exceedingly simple, being merely a thin, leaf-like disc. Face, legs, intestines, etc., are as yet completely wanting. But the five bladders are already quite distinct from one another. [Illustration: FIG. 7.—Embryo of a mammal or bird, in which the five brain-bladders have just commenced to develop. _v._ Fore brain. _z._ Twixt brain. _m._ Mid brain. _h._ Hind brain. _n._ After brain. _p._ Spinal-marrow. _a._ Eye-bladders. _w._ Primitive vertebræ. _d._ Spinal-axis or notochord.] The _first_ bladder, the _fore brain_ (_a_), is in so far the most important that it principally forms the hemispheres of the so-called larger brain (cerebrum), that part which is the seat of the higher mental activities. The more these activities are developed in the series of vertebrate animals, the more do the two lateral halves of the fore brain, or the hemispheres, grow at the expense of the other bladders, and overlap them in front and from above. In man, where they are most strongly developed, agreeing with his higher mental activity, they eventually almost entirely cover the other parts from above (compare Plates II. and III.) The _second_ bladder, the _twixt brain_ (_z_), forms that portion of the brain which is called the _centre of sight_, and stands in the closest relation to the eyes (_a_), which grow right and left out of the fore brain in the shape of two bladders, and later lie at the bottom of the twixt brain. The _third_ bladder, the _mid brain_ (_m_), for the most part vanishes in the formation of the so-called _four bulbs_, a bossy portion of the brain, which is strongly developed in reptiles and birds (Fig. _E_, _F_, Plate II.), whereas in mammals it recedes much more (Fig. _G_, _H_, Plate III.). The _fourth_ bladder, the _hind brain_ (_h_), forms the so-called _little hemispheres_, together with the middle part of the _small brain_ (cerebellum), a part of the brain as to the function of which the most contradictory conjectures are entertained, but which seems principally to regulate the co-ordination of movements. Lastly, the _fifth_ bladder, the _after brain_ (_n_), develops into that very important part of the