CHAPTER XX. 



THE HISTORY OF THE EV^OLUTIOX OF THE EPIDERMIS 

 AXD THE XERVOUS SYSTEM. 



Auimal and Vegetative Organ. systems — Original Eelations of these to the 

 Two Primary Germ-layers. — Sensory Apjjaratus. — Constituents of 

 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 (^Corium). — Appendages of the Epi- 

 dermis : Skin-glands (Sweat-glands, Tear-glands, Sabaceous 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. — Centi-al 

 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 Oerehrurn), Twixt-brain ("Centre 

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

 helium), 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." 



" Hnrdly any parv of the bodily frame, then, could be found better 

 Dalculated to illustrate the truth that the structural differences between 

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



