THE ORGANS OP THE OUTER GERM-LAYER. 427 



chapter, is shown to be carried out in a special manner on the 

 hemispheres of the cerebrum and cerebellum inclusive of the 

 vermiform process, that is to say, on the two parts of the brain 

 which are covered with a gray cortex. That the functional capacity 

 of the cerebrum and cerebellum depends upon the extent of the gray 

 cortex and the regularly arranged ganglionic cells in it, is to be 

 concluded from a large number of phenomena. In this way is 

 explained the very extensive increase of surface which is brought 

 about in the cerebrum and cerebellum by means of somewhat 

 different processes of folding. In the cerebrum broad ridges (gyri) 

 arise from the medullary layer of the hemispheres (centrum semi- 

 ovale), which, running in meandering convolutions, produce the 

 characteristic relief of the surface (fig. 256). In the cerebellum the 



Fig. 240. lateral view of the brain of a human embryo from the first half of the fifth month r 



after MIHALKOVICS. Natural size. 

 stl, Frontal lobe ; schei.l, parietal lobe ; hi, occipital lobe ; schl.l, temporal lobe ; Sy.g, fissure of 



SYLVIUS ; rn, olfactory nerve ; kh, cerebellum ; &;, pons; mob, medulla oblongata. 



numerous ridges proceeding from the medullary nucleus are narrow, 

 arranged parallel to one another, and provided with smaller accessory 

 (secondary and tertiary) ridges, so that the cross section of the 

 cerebellum presents an arborescent figure (arbor vitae). 



If, after these preliminary remarks, we take under consideration 

 the metamorphoses of the five vesicles, we may distinguish on each, 

 as MIHALKOVICS has done in his monograph of the development of 

 the brain, four regions : floor, roof, and two lateral parts. We shall 

 begin our description with the fifth vesicle, because in its structure 

 it approaches most closely to the spinal cord. 



(1) Metamorphosis of the Fifth Brain-Vesicle. 

 The fifth brain-vesicle exhibits in different Vertebrates at the 

 beginning of development (in the Chick on the second and third 



