NERVOUS SYSTEM OF THE VERTEBRATA. 



349 



shaped tube the walls of which are of a nearly uniform thickness, 

 though the roof and Hoor are somewhat thinner than the sides. It 

 forms a direct continuation of the spinal cord, into which it passes 

 without any sharp line of demarcation. The ventricle it contains is 

 known as the fourth ventricle. 



The sides becime in the chick marked by a series of transverse 

 constrictions, dividing it into lobes, which are somewhat indefinite in f 

 number. The first of these remains permanent, and its roof gives rise I 

 to the cerebellum. It is uncertain whether the other constrictions have kf, 

 any morphological significance. More or less similar constrictions are 

 present in Teleostei. In Elasmobranchii the medulla presents on its 

 inner face at a late period a series of lobes corresponding with the roots 

 of the vagus and glossopharyngeal nerves, and it is possible that the 

 earlier constrictions may potentially correspond to so many nerve-roots. 



Throughout the Vertebrata an anterior lobe of the hind-brain 

 becomes very early marked off, so that the primitive hind-brain 

 becomes divided into two regions which may be conveniently spoken 

 of as the cerebellum (figs. 247 and 248, ch) and medulla oblongata. 

 The floor of these regions is quite continuous and is also prolonged 

 without any break into the floor of the mid-brain. 



AOA 



HIND-BRAIN OF A ChICK AT THE END OF THE 



Fig. 249. Section through the 

 third day of incubation. 



IV. Fourth ventricle. The section shews the very thin roof and thicker sides of 

 the ventricle. Ch. Notochord; CV. Anterior cardinal vein; CC. Involuted auditory 

 vesicle; CC points to the end which will form the cochlear canal; RL. Eecessus 

 labyrinth! (remains of passage connecting the vesicle with the exterior) ; hy. Hypoblast 

 lining the alimentary canal ; AO., AOA. Aorta, and aortic arch. 



The posterior section of the hind-brain, which forms the medulla, 

 undergoes changes of a somewhat complicated character. In the first 



