442 EMBRYOLOGY. 



the middle of the lower margin of each hemisphere. The part ot 

 the wall which is thus depressed becomes considerably thickened 

 (figs. 243, 251 cs, and 254 st), and forms an elevation on the floor of 

 the cerebrum projecting into its cavity, the corpus striatum, in which 



several nuclei of gray matter are developed (the nucleus caudatus, the 

 nucleus lentiformis, and the claustrum). Inasmuch as the elevation 

 lies at the base of the brain and forms the direct forward and lateral 



continuation of the optic thalamus, it is regarded as belonging to 

 the brain-stalk, and is distinguished as the stalk part of the cerebral 

 hemispheres in distinction from the remaining portion or mantle part. 



The outer surface of the stalk part can bo seen from the outside for 

 a time, as long as the Sylvian fissure is still shallow (fig. 252 Sy.g), 



echei.l 



mob 



Fig. 252. Lateral view of the brain of a human embryo during the first half of the fifth month, 



after MIHALKOVICS. Natural size. 

 stl, Frontal lobe ; schei.l, parietal lobe ; hi, occipital lobe ; scld.l. temporal lobe; Sy.g, fissure 



of SYLVIUS ; ?, olfactory nerve ; kh, cerebellum ; br, por.s ; mob, medulla oblongata. 



but it then becomes entirely overgrown and hidden by the edges of 

 the gradually deepening fissure. Later this surface acquires in the 

 embryo several cortical furrows and becomes the island of REIL 

 (insula Reilii), or the central lobe (Stammlappen). 



The mantle portion, as it enlarges, spreads out uniformly around 

 the island of REIL, as though about a fixed point, and surrounds it 

 in the form of a half -ring open below (fig. 252) ; on this account it 

 nas received the name ring-lobe. Even now the regions of the four 

 chief lobes into which the convex surface of each hemisphere is 

 subsequently divided can readily be distinguished, although they are 

 not yet sharply limited. The end of the half -ring which is directed 

 forward and lies above the fissure of SYLVIUS (Sy.g} is the frontal 

 lobe (stl) ; the opposite end, which embraces the fissure behind and 



