50 



ANATOMT OF THE CENTEAL XEETOTJS SYSTEM. 



infolding from the dorsal surface. The hemispheres are not developed in 

 the sturgeon; but, even if they were, they would not be shown in Fig. 18, 

 because the section is median. 



In mammals, the hemispheres, which are at first very insignificant 

 appearing structures, soon grow enormously, bending posteriorly and thus 

 covering gradually most of the other vesicles. Finally they rest like a 

 mantle over the Thalamencephalon, the Mesencephalon (Corpora Quadri- 

 gemina), and the Metencephalon (Cerebellum and Pons). 



Notwithstanding the turning back of the cerebral hemispheres, the 



Fig. 20. — Median longitudinal section through the brain of a five-weeks' 

 human embryo. (After His.) 



cavities of the various brain-vesicles, later called ventricles, retain their 

 communication with each other. 



Thus from the primitive forebrain- vesicle two structures are developed: 

 Prosence-phalon, or cerebrum {Vorderhirn), and, from that part of the primi- 

 tive forebrain-vesicle which is not divided into hemispheres, the Thalamen- 

 cephalon (Zwischenhirn). 



In all mammals the walls of the hemispheres begin to grow thicker at 

 this stage. But one soon discovers that that process is carried on by no 

 means equally. Fear the base are located the olfactory lobes as thick-walled 

 masses and posterior to them, also basal, the great corpora striata. These 



