THE BRAIN.—At an early stage in its embryonic history the {368}brain consists of three simple vesicles, the _fore_-, the _mid_-, and the _hind-brain_, the first of which lies in front of the anterior end of the notochord and is therefore pre-chordal in position. As development proceeds the walls of the vesicles undergo local thickenings, or they give rise to hollow paired or median outgrowths, and by one or other of these methods the different parts of the complex adult brain are evolved, while the original cavities of the vesicles or of their outgrowths persist as a continuous system of epithelium-lined spaces or "ventricles."[439] The fore-brain is remarkable for the number and importance of the parts to which it gives rise. First, it bulges out in front into a hollow vesicle, the _prosencephalon_, leaving the rest of the fore-brain as the _thalamencephalon_ or _diencephalon_ (Fig. 210). The cavity of the prosencephalon is the _prosocoele_, and a pair of thickenings in its floor form two basal ganglia or _corpora striata_. In many Fishes the prosencephalon retains this simple vesicular condition, in which case the roof or _pallium_ is usually epithelial and non-nervous; but in others two hollow lobes grow out from it in front and give rise to two _cerebral hemispheres_ or _parencephala_.[440] Both contain extensions of the prosocoele, the _paracoeles_ or _lateral ventricles_, from the floor of which the corpora striata now project. The prolongation of the pallium forming the roof of the lateral ventricles either remains partially epithelial, or it may acquire a wholly nervous structure and thicken to an extent which differs greatly in different Fishes. With the formation of the hemispheres the prosencephalon and its prosocoele become of secondary importance, and may cease to be recognisable as distinct from the thalamencephalon and its ventricle. The lateral ventricles then appear to communicate directly with the third ventricle by two apertures, the _foramina of Munro_. The forward growth of the brain is completed by the development of two hollow lobes, the _olfactory lobes_ or _rhinencephala_, each of which contains a ventricle or _rhinocoele_ communicating behind with the prosocoele, or, if hemispheres are present, with the corresponding lateral ventricle.