862 



DEVELOPMENT 



[CH. LTX. 



vesicle the two primary optic vesicles are budded off laterally : their 

 further history will be traced in the next section. Somewhat later 

 the same vesicle divides into two, and thus the telencephalon and 

 diencephalon are formed. 



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

 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. 660). 



me 



Fio. 660. Early stages in development of human brain (magnified). 1, 2, 3, are from an embryo about 

 seven weeks old ; 4, about three months old. m, Middle cerebral vesicle (mesencephalon) ; c, cere- 

 bellum ; TO o, medulla oblongata ; i (in tig. 3), diencephalon ; A, telencephalon ; i', infundibulum ; 

 fig. 3 shows the several curves which occur in the course of development ; fig. 4 is a lateral view, 

 showing the great enlargement of the cerebral hemispheres which have covered in the thalami, 

 leaving the optic lobes, m, uncovered. (Kolliker.) 



N.B. In fig. 2 the line t terminates in the right hemisphere ; it ought to be continued into the 

 diencephalon. 



(1.) A sharp bend of the whole cerebral mass downwards round the 

 end of the notochord, by which the anterior vesicle, 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 beneath the rudimentary cere- 

 bellum 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 : 



