758 



DEVELOrMEXT OF THE NERVOUS SYSTEM. 



The two optic thalamic fonned from the posterior and outer part of the anterior 

 vesicle, consist at first of a single hollow sac of nervous matter, the cavity of 

 ■which communicates on each side in front with the anterior of the commencing 



Fig. 559. 



Fig. 559. — Remimagrammatic Views of the Inner Surface op the Right Cerebraii 

 Hemisphere of the Fcetal Brain at various stages of development (from 

 Schmidt). 



1, 2, and -3, are from foetuses of the respective ages of eight, ten, and sixteen M'eeks ; 

 4, from a foetus of six months, a, lamina terminalis or imrt of the first primary vesicle 

 which adheres to the sella turcica ; b, section of the cerebral i)eduncle as it passes into 

 the thalamus and corpus striatum ; the arched line which surrounds this bounds the gi-eat 

 cerebral fissure ; c, anterior part of the fornix and the sej)tum lucidum ; d, inner part 

 of the arch of the cerebrum, afterwards the hippocampus major and posterior part of 

 the fornix ; (, corpus callosum very short in 3, elongated backwards in 4 ; in 4, /, the 

 superior marginal convolution ; /', fronto-parietal fissure ; g, gyrus fornicatus ; p', the 

 internal vertical fissure descending to meet the fissure of the hippocampus ; I, olfactory 

 bulb ; F, P, 0, T, frontal, parietal, occipital and temporal lobes. 



cerebral hemispheres, and behind with that of the middle cephalic vesicle 

 (corpora quadrigemina). Soon, however, by increased deposit taking jilace in 



Fig. 560. B 



560. — The Surface of the Fcetal Brain at Six Months (from R. Wagner). 



This figure is intended to show the commencement of the formation of the principal 

 fissures and convolutions. A, from above ; B, from the left side. F, frontal lobe ; P, 

 parietal ; O, occipital ; T, temporal ; a, q, a, slight appearance of the several frontal 

 convolutions ; s, the Sylvian fissure ; s', its anterior division ; within it, C, the central 

 lobe or convolutions of the island ; r, fissure of Rolando ; p, the vertical fissure (external 

 part). 



