46 THE EVOLUTION OF MAN 



over the rest of the brain tube, and ultimately forming the cere- 

 brum, which is so large in man in comparison with the lower 

 animals. All of this sac, except the lower part, as well as the 

 parts of the adult brain formed from it, is known as the mantle, 

 or pallium (Fig. 16), the olfactory portion of which is the 

 archipallium since in evolution it is the oldest part of the brain, 

 while the rest, being a more recent addition, is called the neo- 

 pallium. The latter is most extensively developed in man 

 and forms the major part of the cerebrum. 



The early condition of the neopallium in man represents 

 about the extent of the pallium in the adult fish (Fig. 16). 

 As it grows further backward it represents first the extent of 

 the pallium in the next higher class of vertebrates, the amphib- 

 ians, and later the extent in reptiles. Finally, as it begins to 

 cover the cerebellum, we have the extent of the pallium as 

 found in lower mammals and when it covers the cerebellum 

 completely we have the human pallium. Thus we see that the 

 pallium in its development in man passes through successively 

 the various stages represented in the adult forms of the dif- 

 ferent vertebrates, starting with the fish and terminating with 

 the mammalian type (Fig. 16). Similarly the structural unit 

 of the nervous system, the nerve cell, or neurone (Fig. 17), 

 passes in its development in man from the very simple neurone 

 of the fish through the increasingly complex forms of the vari- 

 ous vertebrates to its greatest complexity in man. 



The sensory nerves (Fig. 18), which connect the skin and 

 the various peripheral sense organs to the central nervous sys- 

 tem, develop by a separation of serial groups of cells from the 

 dorsal portion of the neural folds. From each of the cells in 

 these groups two processes grow out. One grows into the 

 central nervous system and the other pushes its way among 

 the cells of the embryo until it reaches the sensory structures 

 at the periphery. The motor nerves (Fig. 18) which control 

 the muscles, on the contrary, grow out from cells situated in 



