CHAPTER XXIII. 

 DEVELOPMENT OP THE INTESTINAL SYSTEM. 



The Primitive Intestine of the Gastrula. Its Homology, or Morphological 

 Identity in all Animals (excepting the Protozoa). Survey of the 

 Structure of the Developed Intestinal Canal in Man. The Mouth- 

 cavity. The Throat (pharynx). The Gullet (oesophagus). The Wind- 

 pipe (trachea) and Lungs. The Larynx. The Stomach. The Small 

 Intestine. The Liver and Gall-bladder. The Ventral Salivary Gland 

 (pancreas). The Large Intestine. The Eectum. The First Rudiment of 

 the Simple Intestinal Tube. The Gastrula of the Amphioxus and of 

 Mammals. Separation of the Germ from the Intestinal Germ Vesicle 

 (Gastrocystis). The Primitive Intestine (Protogaster) and the After 

 Intestine (Metagaster). Secondary Formation of the Mouth and Anus 

 from the Outer Skin. Development of the Intestinal Epithelium from 

 the Intestinal-glandular Layer, and of all other parts of the Intestine 

 from the Intestinal-fibrous Layer. Simple Intestinal Pouch of the 

 Lower Worms. Differentiation of the Primitive Intestinal Tube into a 

 Respiratory and a Digestive Intestine. Gill-intestine and Stomach- 

 Intestine of the Amphioxns and Ascidian. Origin and Significance of 

 the Gill-openings. Their Disappearance. The Gill-arches and the Jaw- 

 skeleton. Formation of the Teeth. Development of the Lungs from the 

 Swim-bladder of Fish. Differentiation of the Stomach. Development 

 of the Liver and Pancreas. Differentiation of the Small and Large 

 Intestines. Formation of the Cloaca. 



" Cautious people require us to confine ourselves to gathering materials, 

 and to leave it to posterity to raise a scientific structure from those 

 materials ; because only in that way can we escape the ignominy of having 

 the theories we believed in overthrown by the advance of knowledge. The 

 unreasonableness of this demand is apparent enough from the fact that 



