DEVELOPMENT OF THE BEAIN AND GANGLIA. 



53 



well defined, it has the form shown in Fig. 23. It has grown out pos- 

 teriorly and has also hent ventrally. Where, within the cerebral caTity, the 

 corpus striatum is developing the outer wall is less expanded than in the 

 other parts of the cerebrum. 



In that place a relative retardation of the growth of the cerebral wall 

 leaves a deep fissure, the Fossa Sylvii. 



A frontal loie, an occipital lobe, and a parietal lobe may now be easily 

 distinguished upon the hemispheres. That part of the hemispheres below 

 the Sylvian fossa is called the temporal lobe. 



The hemispheres are still hollow within and the ventricular cavities 

 conform naturally to the general cerebral form. Those parts of the ven- 

 tricle which extend into the frontal, occipital, and temporal lobes have 

 been called the anterior, posterior, and inferior horns, respectively. At this 

 stage of development the median wall of the hemisphere demands our 



Fig. 23. — The brain of a human fcetua of the fourth month. 



especial interest. As before mentioned, its ventral margin passes into the 

 epithelium of the plexus chorioideus. This is the case also when the wall 

 bends down with the temporal lobe. From the base of the lamina terminalis 

 to the apex of the temporal lobe, the sickle-shaped fornix locates this 

 margin. In the anterior part of the brain the corpus callosum is developed 

 dorsal to the fornix. The commissural fibers of which the corpus callosum 

 is composed determine a surface which forms an acute angle with the fornix 

 (see Fig. 24). 



That portion remaining between corpus callosum and fornix, and which 

 consists of the two thin laminse of the primitive hemispherical division- 

 wall {primdre scheidewand. Fig. 22) is called the septum pellucidum. These 

 are important structures, which should be carefully studied in the accom- 

 panying figures. 



iN'ote carefully the anatomical relations shown in Fig. 22. In the base 



