CHAPTEK XVII. 



DEVELOPMENT OF THE ALIMENTARY SYSTEM, THE RESPIRATORY 

 SYSTEM, AND THE FACE. 



First appearance of the intestinal canal Formation of the mesentery Forma- 

 tion of the stomach Development of the large intestine Appearance of 

 the intestinal villi Formation of the pharynx and oesophagus Develop- 

 ment of the anus The liver, pancreas, and spleen Development of the 

 respiratory system Development of the face Visceral arches, and their 

 development Malformations of the face Development of the teeth. 



THE intestinal canal is the first formation of the aliment- 

 ary system. As we have seen in Chapter XVL, this is at 

 first open in the greatest part of its extent, presenting, at 

 either extremity of the longitudinal gutter, in front of the 

 spinal column, a rounded, blind extremity, which is closed 

 over in front for a short distance. The closure of the ab- 

 dominal plates then extends laterally and from the two ex- 

 tremities of the intestine, until w r e have only the opening re- 

 maining for the passage of the umbilical cord and the pedicle 

 of the umbilical vesicle. There is at first an open communi- 

 cation between the lower part of the intestinal tube and the 

 allantois, which forms the canal known as the urachus ; but 

 that portion of this communication which remains enclosed in 

 the abdominal cavity becomes separated from the urachus, is 

 dilated, and eventually forms the urinary bladder. When the 

 bladder is first shut off, it communicates with the lower por- 

 tion of the intestine, which is called the cloaca ; but it finally 

 loses this connection, and presents a special opening, the 

 urethra. 



As development advances, the intestine grows rapidly in 



