_Brain._--It is customary to divide the brain into three main regions, fore-, mid-, and hind-brain, as in the most familiar vertebrates there is frequently seen in the embryo a division of the primitive brain dilatation into three vesicles lying one behind the other. A consideration of the development of the brain in the various main groups of vertebrates shows that these divisions are not of equal importance. In those archaic groups where the egg is not encumbered by the presence of a large mass of yolk it is usual for the brain to show in its early stages a division into two main regions which we may term the primitive fore-brain or cerebrum and the primitive hind-brain or rhombencephalon. Only later does the hinder part of the primitive fore-brain become marked off as mid-brain. In the fully developed brain it is customary to recognize the series of regions indicated below, though the boundaries between these regions are not mathematical lines or surfaces any more than are any other biological boundaries:-- Rhombencephalon (Hind-brain) / Myelencephalon (Medulla oblongata). \ Metencephalon (Cerebellum). / Mesencephalon (Mid-brain). Cerebrum (Primitive Fore-brain) < Thalamencephalon (Diencephalon). \ [Hemispheres (Telencephalon).] The myelencephalon or medulla oblongata calls for no special remark, except that in the case of _Torpedo_ there is a special upward bulging of its floor on each side of the middle line forming the electric lobe and containing the nucleus of origin of the nerves to the electric organ. [Illustration: A and B from Wiedersheim, by permission of Gustav Fischer. FIG. 29.--Brain of _Scyllium_ (A), _Salmo_ (B) and _Lepidosiren_ (C). The three figures are not drawn to the same scale. cer, Cerebellum. c.h, Cerebral hemisphere. th, Thalamencephalon. f.b, Primitive fore-brain (in B the line points to the thickened wall of the fore-brain, the so-called "basal ganglia"). G.p, Pineal body. m.b, Roof of mid-brain, optic lobes, _tectum opticum_. o.l, Olfactory lobe. IV.v, Fourth ventricle.] The cerebellum occurs in its simplest form in lampreys and Dipnoans (fig. 29, C), where it forms a simple band-like thickening of the anterior end of the roof of the hind-brain. In Selachians it is very large and bulges upwards, forming a conspicuous organ in a dorsal view of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is also large. It projects back as a great tongue-like structure over the roof of the fourth ventricle, while in front it dips downwards and projects under the roof of the mid-brain forming a highly characteristic _valvula cerebelli_. A _valvula cerebelli_ occurs also in ganoids, while in the Crossopterygians a similar extension of the cerebellum projects backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).