433 



THE CEREBRAL HEMISPHERES. 



. f A 



FIG. 257. DIAGRAMMATIC LONGITUDI- 

 NAL HORIZONTAL SECTION THROUGH THE 

 FORE-BRAIN. 



3-z>. third ventricle ; Iv. lateral ventricle ; 

 It. lamina terminalis ; ce. cerebral hemi- 

 sphere ; op.th. optic thalamus. 



The first change which takes place consists in the roof 

 growing out into two lobes, between which a shallow median 

 constriction makes its appearance (fig. 257). The two lobes 

 thus formed are the rudi- 

 ments of the two hemi- 

 spheres. The cavity of each 

 of them opens by a widish 

 aperture into the vestibule 

 at the base of the cerebral 

 rudiment, which again opens 

 directly into the cavity of 

 the third ventricle (3 v). 

 The Y-shaped aperture thus 

 formed, which leads from 

 the cerebral hemispheres 

 into the third ventricle, is 

 the foramen of Munro. The 

 cavity (Iv) in each of the 

 rudimentary hemispheres is 

 a lateral ventricle. The part of the cerebrum which lies between 

 the two hemispheres, and passes forwards from the roof of the 

 third ventricle round the end of the brain to the optic chiasma, 

 is the rudiment of the lamina terminalis (figs. 257 It and 255 trm). 

 Up to this point the development of the cerebrum is similar in 

 all Vertebrata, but in some forms it practically does not proceed 

 much further. 



In Elasmobranchii, although the cerebrum reaches a con- 

 siderable size (fig. 254 cer], and grows some way backwards over 

 the thalamencephalon, yet it is not in many forms divided into 

 two distinct lobes, but its paired nature is only marked by 

 a shallow constriction on the surface. The lamina terminalis in 

 the later stages of development grows backwards as a thick 

 median septum which completely separates the two lateral 

 ventricles 1 (fig. 263). 



There are, it may be mentioned, considerable variations in 



1 A comparison of the mode of development of this septum with that of the septum 

 lucidum with its contained commissures in Mammalia clearly shews that the two 

 structures are not homologous, and that Miklucho-Maclay is in error in attempting to 

 treat them as being so. 



