GENERATION AND DEVELOPMENT. 



289 



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. 446, 3). 

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

 of the notochord, by which the anterior vesicle, which was the highest of 



VIO 



FIG. 446. 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 (metencephalon); c, 

 cerebellum; mo, medulla oblqngata; i, thalamencephalon; h, hemispheres; i', infundibulum. Fig. 

 3 shows the several curves which occur in the course of development. Fig. 4 is a lateral view, show- 

 ing 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 i terminates in the right hemisphere; it ought to be continued into the 

 thalamencephalon. 



the three, is bent downward, and the middle one comes to occupy the 

 highest position. (2.) A sharp bend, with the convexity forward, which 

 runs in from behind beneath the rudimentary cerebellum separating it 

 from the medulla. 



Thus, five fundamental parts of the foetal brain may be distinguished, 

 which, together with the parts developed from them may be presented in 

 the following tabular view. 



4 



Table of Parts Developed from Fundamental Parts of Brain. 



{Cerebral hemispheres, corpora, 

 striata, corpus callosum, for- 

 nix, lateral ventricles, olfac- 

 tory bulb (Rhinencephalon). 

 ( Thalami optici, pineal gland, pit- 

 < uitary body, third ventricle, 

 ( optic nerve (primarily). 



I. Anterior 

 Primary 

 Vesicle. 



1. Prosencephalon. 



2. Thalamencephalon 

 (Diencephalon. ) 



VOL. II. 19. 



