THE CEBEBEUM OR PROSENCEPHALON. 



167 



higher vertebrates, the more anterior sections always show an approximately 

 oval outline on whose base the corpus striatum lies (see Fig. 103); farther 

 posterior (Fig. 101) one finds the septum on the inner wall, which loses its 

 previously simple features; and finally one comes (Fig. 99) to the place 

 where the mantle merges into the choroid plexus and becomes purely epithe- 

 lial (Fig. 120). At this point in the base of the brain the boundary between 

 the forebrain and the interbrain is usually reached, and one sees upon the 

 sections the tracts which unite these two segments of the brain. 



The author assumes that this brief description illustrated by the figures 

 has sufficiently familiarized the reader with the outer form of the cerebrum, 

 and will now describe the structure of certain parts of the mantle. 



As the cross-sections of the whole amphibian brain is strikingly like 

 the embryonic brains of the other vertebrates, so even in the forebrain a 

 structure will be found which always recurs in higher vertebrates in the 



Fig. 116. — Section through the mantle of- a frog-brain. 

 Pedro EamOn y Cajal.) 



(After 



embryonic period. Thus, one can, in a section through the cerebral wall, 

 usually differentiate only two layers : an inner one rich in cells and an outer 

 •one sparsely filled with cells, At several places in the mantle, — ^near the 

 olfactory apparatus, for example, — in the anterior part of the Eegio parolf ac- 

 toria, and then in the postero-median region of the mantle the inner layer 

 shows especial projections: evidently a greater development of the cells 

 which constitute it. Good sections well stained show that the inner layer 

 next to the ventricle is formed of epithelial cells, which send their long 

 branching processes up through the entire mantle to the outer surface, thus 

 making a frame-work for the brain-mantle (see Fig. 116, the left margin). 

 This frame-work, formed of the terminal processes of epithelial cells, is, be- 

 sides this, present, also, in all portions of the brain posterior to this, and is 

 persistent even in the reptilia. In birds and mammals a large part of the 

 terminal processes disappear in post-embryonal life. Then, farther outward 



