122 THE FEOG. 



The medulla oblongata undergoes less change than any 

 other part of the brain. 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, the roof of the medulla oblongata is thin ; 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, consisting 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 choroid 

 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 fourth ventricle, 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. 



The cerebellum is 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 transverse band, 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 very 

 small as compared with its condition in most other Vertebrates. 



The mid-brain does not undergo very great changes. Its 

 floor remains thin in the actual median plane ; but immediately 



