-t> 



368 FISHES CHAP. 



"brain consists of three simple vesicles, the fore-, the mid-, and 

 the liind-lrain, 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 out- 

 growths, 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 con- 

 tinuous system of epithelium-lined spaces or " ventricles." ^ 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 23rosence23lialon, leaving the rest of the fore- 

 brain as the thcdamcncephcdon or dienceiolialon (Fig. 210). The 

 cavity of the prosencephalon is the prosocoele, and a pair of 

 thickenings in its floor form two basal ganglia or coiyora 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 cerehrcd hemispheres or paren- 

 cej^hala." Both contain extensions of the prosocoele, the para- 

 coeles 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 epi- 

 thelial, 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 com- 

 municate directly with the third ventricle by two apertures, the 

 foramina of Munro. The forward growth of the brain is com- 

 pleted by the development of two hollow lobes, the olfactory hies 

 or rhinencephcda., each of which contains a ventricle or rhinocoele 

 communicating behind with the prosocoele, or, if hemispheres 

 are present, with the corresponding lateral ventricle. Scarcely 



^ For the nomenclature of the brain and its cavities see T. J. Parker, Nature, 

 XXXV. 1886, p 208 ; and Parker and Haswell, Text-Book of Zoology, London, 1897, 

 ii. p. 94. 



^ It is possible that the prosencephalon is merely the bulging anterior part of 

 the thalamencephalon ; if this be so the hemispheres are really paired outgrowths 

 from the thalamencephalon. 



