368 FISHES CHAP. 
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 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 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 palliwm 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 paren- 
cephala.” 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 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. Scarcely 
1 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, 
li. p. 94. 
2 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 thalamencephalJon. 
