XII.] THE CEREBRAL HEMISPHERES. 377 



The two lobes thus formed are the rudiments of the 

 two hemispheres. The cavity of each of them opens 

 by a widish aperture into a cavity at the base of the 

 cerebral rudiment, which again opens directly into the 

 cavity of the third ventricle (3 v). The Y-shaped aper- 

 ture thus formed, which leads from the cerebral hemi- 

 spheres into the third ventricle, is the foramen of 

 Monro. The cavity (Iv) in each of the rudimentary 

 I 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 below, 

 is the rudiment of the lamina terminalis (Figs. 121 It 

 and 128 trm). Up to this point the development of 

 the cerebrum is similar in all Vertebrata, and in some 

 forms it practically does not proceed much further. 



The cerebral hemispheres undergo in Mammalia the 

 most complicated development. The primitive un- 

 paired cerebral rudiment becomes, as in lower Ver- 

 tebrates, bilobed, and at the same time divided by the 

 ingrowth of a septum of connective tissue into two 

 distinct hemispheres (Figs. 125 and 124/" and 122 i). 

 From this septum is formed the falx cerebri and other 

 parts. 



The hemispheres contain at first very large cavities, 

 communicating by a wide foramen of Monro with the 

 Ithird ventricle (Fig. 124). They grow rapidly in size, 

 and extend, especially backwards, and gradually cover 

 the thalamencephalon and the mid-brain (Fig. 122 i,f). 

 The foramen of Monro becomes very much narrowed 

 and reduced to a mere slit. 



The walls are at first nearly uniformly thick, but 



