288 STRUCTURE OF THE VERTEBRATES 



vesicles which form the optic stalks and the retina of each eye; 

 and dorsally there are two pockets, the cerebral hemispheres, the 

 ventricle of each connected with the original median ventricle 

 of the forebrain by a wide joramen of Munro. 



The forebrain was probably given over entirely to the ol- 

 factory function in the primitive ancestors of the vertebrates, 

 for in the cyclostomes this function occupies practically all of 

 this region; but in the elasmobranch fishes two lateral nuclei 

 develop, the corpora striata. The dorsal covering, or pallium, is 

 non-nervous, and functions only as a roof for the ventricle. Dur- 

 ing evolution the olfactory function becomes progressively less 

 important and the development of the cerebral hemispheres be- 

 comes more and more prominent. 



Cerebral Hemispheres. The telencephalon of the dogfish has 

 the nuclei enclosed by the peripheral nerve fibers and non-nerv- 

 ous tissues. The amphibia keep the same general relationship 

 with two advances, (1) a material increase in the size of the 

 hemispheres, and (2) the invasion of the ventricular side of the 

 hemispheres by nerve cells from the lateral nuclei. Both tenden- 

 cies increase in the turtles and other lower reptiles. The nerve 

 cells cover the inner surface of the pallium and collect along the 

 median line as two distinct nuclei. The dorsal and median 

 growth of the hemispheres has pushed them close together, so 

 that the telencephalon is divided by a deep fissure resembling 

 the fissure of the spinal cord. The cerebral pallia so far described 

 are grouped together as archipallia, the primitive pallium with 

 non-nervous tissue on the outside. 



The neopallium is found first in the crocodilia. In these ani- 

 mals the nerve cells of the ventricular side migrate through the 

 pallium to the outer surface and form a primitive cerebral cortex. 

 A more extensive cortex is found in the birds and monotremes. 

 In both groups the cerebral hemispheres are large, covered with 

 a thick cortex, but perfectly smooth. Further progress is found 

 in the marsupials. The cortical layer of cells multiplies more 

 rapidly than the pallium grows, and the cortex becomes slightly 

 creased and folded into convolutions. The evolution of the mam- 

 malian brain is an advance along the two lines indicated, growth 

 in absolute size of the hemispheres, and multiplication of cortical 

 cells. 



