X CONTENTS. 



PAGI 



Layer and the Skin-fibrous Layer. The Entodenn or Intestinal 

 Layer gives rise to the Intestinal-fibrous Layer and the Intestinal- 

 glandular Layer 184 



CHAPTER IX. 



THE VERTEBRATE NATURE OP MAN. 



Relation of Comparative Anatomy to Classification. The Family -rela- 

 tionship of the Types of the Animal Kingdom. Different Signi- 

 ficance and Unequal Value of the Seven Animal Types. The 

 Gastrcea Theory, and the Phylogenetic Classification of the Animal 

 Kingdom. Descent of the Gastraea from the Protozoa. Descent 

 of Plant-animals and Worms from the Gastrasa. Descent of 

 the Four Higher Classes of Animals from Worms. The Verte- 

 brate Nature of Man. Essential and Unessential Parts of the 

 Vertebral Organism. The Amphioxus, or Lancelet, and the Ideal 

 Primitive Vertebrate in Longitudinal and Transverse Sections. 

 The Notochord. The Dorsal Half and the Ventral Half. The 

 Spinal Canal. The Fleshy Covering of the Body. The Leather- 

 skin (corium). The Outer-skin (epidermis). Body-cavity (cceloma). 

 The Intestinal Tube. The Gill-openings. The Lymph- vessels. 

 The Blood-vessels. The Primitive Kidneys and Organs of He- 

 production. The Products of the Four Secondary Germ-layers ... 241 



CHAPTER X. 



THE CONSTRUCTION OF THE BODY FROM THE GERM- 

 LAYERS. 



The Original (Palingenetic) Development of the Vertebrate Body from 

 the Gastrnla. Relation of this Process to the Later (Kenogenetic) 

 Germination, as it occurs in Mammals. The most important act in 

 the Formation of the Vertebrate. The Primary Germ-layers, and 

 also the Secondary Germ-layers, which arise by Fission of the Prima- 

 ries, originally form Closed Tabes. Contemporaneously with the 

 Completion of the Yelk-sac, the Germ-layers flatten, and only later 

 again assume a Tabular Form. Origin of the Disc-shaped Mamma- 

 lian Germ-area. Light Germ-area (area pellucida) and Dark Germ- 

 area (area opaca). The Oval Germ-shield, which afterwards 

 assumes the Shape of the Sole of a Shoe, appears in the Centre of 

 the Light Germ-area (a. pellucida) . The Primitive Streak 



