484 STRUCTURE OF VERTEBRATA. 
process is completed, certain portions of the anterior region 
of the medullary canal grow more rapidly than others, and 
form the three primary brain vesicles. By further processes 
of growth and constriction, these three form the five regions 
of the adult brain. 
When first formed the brain vesicles lie in a straight line, but asa 
consequence, probably, of their rapid and unequal growth, this condition 
is soon lost, and a marked cranial flexure is produced. In the lower 
forms, e.g. Cyclostomata, the flexure is slight, and is corrected later, 
but in the higher types it is very distinct, and causes the marked over- 
lapping of parts so obvious in the adult. 
Fic. 258.—Partial section of a Vertebrate brain (diagrammatic). 
OLF., Olfactory lobe ; CH., cerebral hemispheres ; C., wall of cerebrum 
cut to show ventricle, behind this the figure is that of a median sec- 
tion; PA., parietal organ arising from thalamencephalon; //., 
pineal organ; /VF., infundibulum descending from thalamen- 
cephalon; #., hypophysis; OL., optic lobes; C4., cerebellum ; 
CPL., choroid plexus on roof of fourth ventricle ; AZO., floor of the 
medulla oblongata ; CC., central canal of spinal cord. 
We must now follow the metamorphosis of the primary 
brain vesicles. 
The first vesicle gives rise anteriorly to the cerebral hemi- 
spheres, while the remainder forms the region of the optic 
thalami or thalamencephalon. 
The cerebral hemispheres (prosencephalon or fore-brain) 
are exceedingly important. They predominate more and 
more as we ascend in the scale of Vertebrates, and become 
more and more the seat of intelligence. Except in a few 
cases, the prosencephalon is divided into two parts— 
the cerebral hemispheres—which contain cavities known as 
the lateral ventricles. The two hemispheres are united by 
