332 



Elementary Biology. 



FIG. 189. VENTRICLES OF 

 J HE BRAIN. (Jeffrey Bell.) 



greatest amount of differentiation, viz. the nervous system 

 and by summarising the very remarkable changes undergone 

 by the embryo after it has been hatched. 



The development of the brain. The nervous system 

 of the frog originates somewhat similarly to that of Am- 

 phioxus, save that in the former the neural canal is formed 

 entirely by the laminae dorsales. The anterior blind end 

 of this canal becomes swollen so as 

 to form three so-called cerebral 

 vesicles, the walls of which become 

 differentiated into the fore-, mid-, 

 and hind- brains, while their cavities 

 are represented by chambers already 

 referred to as present in the fully 

 developed brain. From the first- 

 cerebral vesicle a hollow bud is 

 given off on either side to form the 

 cerebral hemispheres, the vesicle 

 itself becoming the cavity of the 

 thalamencephalon ; its cavity is 

 known as the third ventricle. The 

 walls of the second cerebral vesicle 

 differentiate to form the optic lobes ; 

 its cavity becomes a narrow channel 

 T<nown as the iter. The walls of the 

 hind-brain develop into the cere- 

 bellum and medulla oblongata, while 

 its cavity, into which opens the iter 

 anteriorly and the central canal of 



the spinal column posteriorly, is spoken of as the fourth 

 ventricle. 



Metamorphosis. The young of the frog after it has 

 escaped from its vitelline membrane is known as a tadpole, 

 and differs very markedly from the adult frog in the general 

 contour of the body. The head is not well differentiated 

 from the body, and there is a long tail, by the rhythmic 



213 



in, third ventricle ; n, iter ; 

 CH, cerebral hemispheres ; 

 cv, their cavity ; FM, pass- 

 age from third ventricle to 

 cavity of cerebral hemi- 

 sphere ; FB, fore-brain ; MB, 

 mid-brain ; HB, hind-brain. 



