122 THE FROG. 



Ths Tiifiiinna. nTilnngata. undergoes 1'^«f f|iaTigp tTian ajjv 

 ot }iP!r pnr f-. f>f tliA ]ivg.in In the early stages, up to about the 

 time of formation of the mouth, it is the widest part of the 

 brain, but afterwards it is exceeded by both the optic lobes and 

 the cerebral hemispheres. It is continuous posteriorly, without 

 any line of demarcation, with the spinal cord ; while anteriorly 

 it is separated from the mid-brain by a well-marked constriction, 

 deepest dorsally and at the sides. 



From the first, th e roof of the medulla oblongata is t hin ; in 

 the later stages the sides and floor thicken very considerably, 

 while the roof (Figs. 65 and 84) widens out and becomes re- 

 duced to an extremely thin membrane, consistiug of a single 

 layer of pigmented and ciliated epithelial cells, without nervous 

 elements of any kind. 



This thin roof is at first smooth and level ; but about the 

 time of formation of the mouth opening, i.e. in tadpoles of about 

 9 mm. length, the roof becomes thrown into folds (Fig. 65, x'), 

 which become deeper and more pronounced as the tadpole in- 

 creases in size. Lying on this thin roof, and in very close 

 contact with it, is a rich network of blood-vessels, the ^horoi d 

 plexus , which extends between the folds of the roof, and so 

 appears to hang down into the cavity of the medulla, though 

 always in reality separated from this by the thin epithelial roof. 



The cavity of the medulla oblongata, or fog rth ventri cle, is 

 of considerable size : it is wide in front, and tapers gradually 

 towards its hinder end, where it passes into the central canal of 

 the spinal cord. 



T he cerebelliiin i s an inconspicuous structure throughout 

 the early stages of tadpole life. ' Up to the time of the opening 

 of the mouth it can hardly be said to exist (Fig. 64) ; but shortly 

 after this event it appears as a thickening of the roof of the 

 fourth ventricle, in the form of a trans verse ba nd, immediately 

 behind the constriction separating the medulla oblongata from 

 the mid-brain. In the later stages of development it increases 

 gradually in. size (Fig. 89, bl), but even in the adult frog it is ve ry 

 small as compare d wi th its condition in most other V ertebrates. 



Th e mid-brarg d oes no t undergo vei-y great chang es. Its 

 floor remains thin in the actual median plane ; bat immediately 



