420 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



nature, and is a structure which is not of cerebral 

 origin at all, but is derived from the epiblast which 

 lines the cavity of the mouth ; in its primitive con- 

 dition it forms an inpushing towards the lower 

 surface of the brain ; its base becomes solid, and 

 then disappears, so that the ingrowth becomes 

 completely separated off from the layer of cells 

 from which it took its origin. In the lower ver- 

 tebrates it does not, but in mammals it does, become 

 structurally united with the brain. 



In Fishes the brain is always small ; in the pike, 

 for example, it is not more than T ^^th part of the 

 weight of the whole body, whereas in Man it is about 

 g^th of the total weight ; nor does it grow propor- 

 tionately with the growth of the body, or occupy the 

 whole of the cranial cavity. In the Cyclostomata 

 the walls of the cerebral hemispheres become greatly 

 thickened, so much so, indeed, that in Myxine they 

 become quite solid ; the olfactory lobes are propor- 

 tionately large, as is also the pineal gland ; the 

 region of the hind-brain is also of great size, propor- 

 tionately to the rest of the organ. In the Elasmo- 

 branchs the olfactory lobes are often carried 

 forwards on stalks, which are of great length in some 

 sharks ; these lobes may be broken up into smaller 

 lobules. The cerebral hemispheres are proportionately 

 large, and differ greatly in the size of the contained 

 ventricles, or, in other words, in the thickness of their 

 walls ; the surface of these hemispheres is sometimes 

 marked by a few shallow grooves. The cerebellum is 

 of large size, and is often grooved transversely. 



During the process of development the brain 

 vesicles cease to lie in a straight line one behind 

 the other ; as a consequence of this " cranial 

 flexure," the fore-brain lies at a lower plane than 

 the mid- brain, and the long axes of the two are 

 set at an angle to one another. A little later the 



