The foregut extends from the buccopharyngeal membrane to the level of liver bud, which appears as an outgrowth from the developing duodenum during the 4th week of development. The pericardial cavity and septum transversum lie ventral to the fore-gut, while it is flanked on each side by a pericardioperitoneal canal.
Soon an outgrowth, called tra-cheobronchial diverticulum, appears on the ventral aspect of the foregut and, now two parts of the foregut can be distinguished: (1) primitive pharynx, and (2) foregut proper.
The primitive pharynx is the initial portion of the foregut; it is a relatively dilated part extending from the buccopharyngeal membrane to the level of tracheobronchial diverticulum.
The foregut proper is more or less a tubular part extending from the tracheobronchial diverticulum to the level of the liver bud.
Derivatives of the primitive pharynx include oral cavity (some part), tongue, pharynx, middle ear cavity, auditory tube, tonsils, thyroid and parathyroid glands, etc. Development of these structures will be considered in the chapter 10 (dealing with the development of head and neck).
The development of various derivatives of the foregut proper will now be discussed. These derivatives are: (1) esophagus, (2) stomach, (3) duodenum (upto the level of opening of the
bile duct), (4) liver and biliary apparatus, and (5) pancreas.