CHAPTER XX. 



THE HISTORY OF THE EVOLUTION OF THE EPIDERMIS 

 AND THE NERVOUS SYSTEM. 



Animal and Vegetative Organ-systems. — Original Relations of these to tlio 

 Two Primary Germ-layers. — Sensory Apparatus. — Constituents 6i 

 Sensory Apparatus : originally only the Exoderm, or Skin-layer ; after- 

 wards, the Skin-covering specialized from the Nerve-system. — Double 

 Function of the Skin (as a Covering and as Organ of Touch). — Outer 

 Skin (Epidermis) and Leather-skin (CoWitm). — Appendages of the Epi- 

 dermis : Skin-glands (Sweat-glands, Tear-glands, Sebaceous Glands, 

 Milk-glands); Nails and Hair. — The Embryonic Wool-covering. — Hair 

 of the Head and of the Beard. — Influence of Sexual Selection. — Arrange- 

 ment of the Nerve-system. — Motor and Sensory Nerves. — Central 

 Marrow : Brain and Dorsal Marrow. — Constitution of the Human Brain : 

 Large Brain {Cerebrum) and Small Brain {Cerebellum). — Comparative 

 Anatomy of the Central Marrow. — Germ-history of the Medullary-tube. 

 — Separation- of the Medullary-tube into Brain and Dorsal Marrow. 

 — Modification of the Simple Brain-bladder into Five Consecutive Brain- 

 bladders : Fore-brain (Large Brain, or Cerebrum), T wixt-braiu (" Centre 

 of Sight "), Mid-brain (" Four Bulbs "), Hind-brain (Small Brain, or Cere- 

 bellum), After-brain (Neck Medulla). — ^Various Formation of the Five 

 Brain-bladders in the various Vertebrate Classes. — Development of 

 the Conductive Marrow, or " Peripheric Nervous System," 



" Hardly any part of the bodily frame, then, could be found better 

 calculated to illustrate the truth that the structural differences between 

 Man and the highest Ape are of less value than those between tlie highest 



