IEVELOPMENT. 



699 



III. Posterior 

 Primary 

 Vesicle. 



4. Epencephalon. 

 5< M etencephalon. 



f Cerebellum, pons Varolii, 

 < anterior part of fourth ven- 

 ( tricle. 



Medulla oblongata, fourth 

 ventricle, auditory nerve. 

 (Quain.) 



The cerebral hemispheres grow rapidly upwards and backwards, 

 while from their inferior surface the olfactory bulbs are budded off, and 

 the prosencephalon, from which they spring, remains to form the third 

 ventricle and optic thalami. The middle cerebral vesicle (mesencepha- 

 lon) for some time is the most prominent part of the foetal brain, and in 

 Fishes, Amphibia, and Eeptiles, it remains uncovered through life as 

 the optic lobes. But in Birds the growth of the cerebral hemispheres 

 thrusts the optic lobes down laterally, and in Mammalia completely 

 overlaps them. 



In the lower Mammalia the backward growth of the hemispheres 

 ceases as it were, but in the higher groups, such as the monkeys and 



FIG. 484. Side view of foetal brain at six months, showing commencement of formation of the 

 principal fissures and convolutions. F, frontal lobe; P, parietal; O, occipital; T, temporal; a a a, 

 commencing frontal convolutions; s, Sylvian fissure; s', its anterior division; c, within it the central 

 lobe or island of Eeil; r, fissure of Rolando; p, perpendicular fissure. (R. Wagner. ) 



man, they grow still further back, until they completely cover in the 

 cerebellum, so that on looking down on the brain from above, the cere- 

 bellum is quite concealed from view. The surface of the hemispheres 

 is at first quite smooth, but as early as the third month the great Syl- 

 vian fissure begins to be formed (Fig. 483, 4). 



The next to appear is the parieto-occipital or perpendicular fissure ; 

 these two great fissures, unlike the rest of the sulci, are formed by a 

 curving round of the whole cerebral mass. 



In the sixth month the fissure of Rolando appears : from this time 

 till the end of foetal life the brain grows rapidly in size, and the convo- 

 lutions appear in quick succession ; first the great primary ones are 

 sketched out, then the secondary, and lastly the tertiary ones in the sides 

 of the fissures. The commissures of the brain (anterior, middle, and 

 posterior), and the corpus callosum, are developed by the growth of 

 fibres across the middle line. 



