CH. LVIII. 



DEVELOPMENT OF THE BRAIN. 



845 



Somewhat later the same vesicle divides into two, and thus the 

 prosencepkalon and thalamencephalun are formed. 



In the walls of the posterior (third) cerebral vesicle, a thicken- 

 ing appears (rudimentary cerebellum) which becomes separated 

 from the rest of the vesicle by a deep inflection. 



At this time there are two chief curvatures of the brain (fig. 

 664). (i.) A sharp bend of the whole cerebral mass downwards 

 round the end of the notochord, by which the anterior vesicle, 



""" I ; 



Pig. 664. --Early stages in development of human brain (magnified), i, 2, 3, are from an 

 embryo about seven weeks old ; 4, about three months old. m, middle cerebral vesicle 

 (mesencepholon) ; (.cerebellum; m n. medulla oblonguta ; i, thalomencephalon ; k, 

 hemisphere* ; i', infundibulum ; tig. .1 shown the several curvet, .vlii.-h occur in the 

 course of development ; flg. d is a lateral view, showing the great enlargement of the 

 cerebral h'inij>huivs which have covered in the thahuni, leaving the optic lobes, m, 

 uncovered. (Kiilliker.) 

 N.B. In flg. 2 the line i terminates in tlie right hemi.phere ; it ought to be continued 



into the thalamencephalon. 



which was the highest of the three, is bent downwards, and the 

 middle one comes to occupy the highest position. (2.) A sharp 

 bend, with the convexity forwards, which runs in from behind 

 beneath the rudimentary cerebellum separating it from the 

 medulla. 



Thus, five fundamental parts of the foetal brain may be distin- 

 guished, which, together with the parts developed from them, may 

 be presented in the following tabular view : 



