THE FIRST CEREBRAL VESICLE. 



71 



The commissures of the cerebral hemispheres are also formed in the united portion 

 of the mesial walls of the vesicles ; the anterior is the earliest to appear, thus 

 coinciding; with the early appearance of the corpora striata, which it unites in front. 

 The anterior part of the fornix, with its pillars, and the corpus albicans (which is 

 at first single and median) are next formed, followed at a later period by the 

 posterior pillars, which are seen running backwards on each side into the cornu 

 ammonis as soon as this structure becomes distinct. The corpus callosum is the 

 last of the commissures to be formed. Its anterior part appears first, but as the 

 hemispheres extend backwards the formation of the commissure accompanies the 

 backward extension. 



The olfactory lobes are formed as hollow outgrowths from the lower and 

 lateral parts of the hemisphere vesicles (figs. 70, 74). In man they soon show a 

 division into two parts, an anterior and posterior ; of which the latter remains in 

 close connection with the hemisphere vesicle, while the anterior grows out towards 



Fig. 80. THE SURFACE OF THE FCETAL BRAIN AT six MONTHS. (R. "Wagner.) 



This figure shows the formation of the principal fissures. A, from above ; B, from the left side. 

 F, frontal lobe ; P, parietal ; 0, occipital ; T, temporal ; a, a, a, slight appearance of sulci in the 

 frontal lobe ; s, Sylvian fissure ; *', its anterior division ; within it, C, the central lobe ; r, Rolandic 

 sulcus ; p, parieto-occipital fissure. 



Fig. 81. VlEW OF THE INNER SURFACE OF THE 

 RIGHT HALF OF THE FCETAL BRAIN OF ABOUT 



six MONTHS. (Reichert.) 



F, frontal lobe ; P. parietal ; 0, occipital ; 

 T, temporal ; /, olfactory bulb ; //, optic 

 nerve ; fp, calloso- marginal fissure ; p, p', parts 

 of the parieto-occipital fissure ; h, calcarine 

 fissure ; g, g, gyrus fornicatus ; c, c, corpus 

 callosum ; -9, septum lucidum ; /, placed be- 

 tween the middle commissure and the foramen 

 of Monro ; v, in the upper part of the third 

 ventricle ; r', in the back part of the third 

 ventricle ; v", in the lower part of the third 

 ventricle above the infundibulum ; r, reccssus 

 pinealis ; pv, pons Varolii ; Ce, cerebellum. 



the olfactory area of the external epiblast. After the first month these lobes are 

 relatively small in size and their cavities become gradually obliterated, but in some 

 animals, as in the horse, they are large, and their cavities are permanently in 

 communication with the anterior cornna of the lateral ventricles. Their further 

 development is given subsequently (p. 79). 



Formation of the fissures and convolutions. The enlargement of the 

 cranium does not always keep pace with the growth in extent of the walls of the 



