CHAPTER XI. 



GENERAL STRUCTURE AND ARTICULATION OP THE 

 INDIVIDUAL. 



Essential Agreement between the Chief Palingeuetic Germ Processes in the 

 case of Man and in that of other Vertebrates. The Human Body, like 

 that of all Higher Animals, develops from Two Primary and Four 

 Secondary Germ-layers. The Skin-sensory Layer forms the Horn-plate, 

 the Medullary Tube, and the Primitive Kidneys. The Middle Layer 

 (Mesoderm) breaks up into the Central Notochord, the Two Primitive 

 Vertebral Cords, and the Two Side-layers. The latter split up into the 

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

 glandular Layer forms the Epithelium of the Intestinal Canal, and of 

 all its Appendages. Ontogenetic and Phylogenetic Fission of the 

 Germ-layers. Formation of the Intestinal Canal. The Two-layered 

 Globular Intestinal Germ-vesicle of Mammals represents the Primitive 

 Intestine. Head Intestinal Cavity, and Pelvic Intestinal Cavity. 

 Mouth Groove and Anal Groove. Secondary Formation of Mouth 

 and Anus. Intestinal Navel and Skin-navel. Movement of the Primi- 

 tive Kidneys from the Outside to the Inside. Separation of the 

 Brain and Spinal Marrow. Endiments of the Brain-bladders. The 

 Articulation or Metameric Structure of the Body. The Primitive 

 Vertebrae (Trunk-Segments, or Metamera). The Construction and 

 Origin of the Vertebral Column. Vertebral Bodies and Vertebral 

 Arches. Skeleton-plate and Muscle-plate. Formation of the Skull 

 from the Head-plates. Gill-openings and Gill-arches. Sense-organs. 

 Limbs. The Two Front Limbs and the Two Hind Limbs. 



" The occurrence of an internal skeleton in definite local relations to the 

 other organ-systems, and the articulation of the body into homologous 

 segments, are points in the general organization of Vertebrates to which 

 especial weight must be given. This metameric structure is more or less 

 definitely expressed in most of the organs, and as it extends to the axial 

 skeleton, the latter also gradually articulates into separate segments, the 

 vertebrae. The latter, however, must be regarded only as the partial ex- 



