Johnston, Forehrain Vesicle in Vertebrates. 519 



The term hemisphere is applied in the BNA to each half of the 

 telencephalon. It would therefore include the right or left half 

 of all that lies in front of a plane passing behind the interventricular 

 foramina and the chiasma-ridge. It is well known that this portion 

 of the brain is not hemispherical in form in all classes. It is some- 

 what so in cyclostomes, many selachians, amphibians, reptiles, birds 

 and mammals. In Heptanchus, Hexanchus, Chimsera the hemi- 

 sphere is more elongated and the membraneous roof is more exten- 

 sive. In ganoids and teleosts the width of the membraneous roof 

 is greatly exaggerated. The nervous walls are rolled outward so 

 that the membraneous roof is attached along the lateral or even 

 latero-ventral aspect and arches up over the ventricle. This ever- 

 sion of the forebrain walls in teleosts has made the term hemisphere 

 inapplicable in the descriptive sense. However, most of the organs 

 which make up the hemisphere in other forms are present in the 

 teleostean telencephalon and these organs hold the same fundamental 

 morphological relations to one another and to other parts of the 

 brain. Therefore the term hemisphere may be employed throughout 

 the vertebrate series, although in no two classes does the telenceph- 

 alon approach in the same degree the form of a sphere. 



In each hemisphere are represented nervous and membraneous 

 portions. The membraneous portions include the lamina terminalis 

 and the tela chorioidea. The lamina terminalis is supposed to be 

 coextensive with the anterior neuropore, but there is no neuropore 

 in cyclostomes and teleosts and in some other vertebrates (some 

 amphibians at least) the upper border of the neuropore is not marked 

 in the early embryos. Where an unambig-uous recessus neuropor- 

 icus exists it is the clear mark of the dorsal border of the lamina 

 terminalis. Where this landmark is not clear an arbitrary border 

 for the lamina terminalis must be placed at some distance in front 

 of the interventricular foramina. The tela forms the roof from the 

 lamina terminalis to the tela of the diencephalon, from which it is 

 separated by the infolded velum transversum. The term pars supra- 

 neuroporica of the lamina terminalis which was used by Burckhardt 

 (1894) and is used by Edinger for this portion of the brain roof is 

 wholly without justification. 



