THE DIGESTIVE APPARATUS 



1101 



The Development of the Digestive Tube. The primitive digestive tube con- 

 sists of two parts, viz.: (1) the fore-gut, within the cephalic flexure, and dorsal 

 to the heart; and (2) the hind-gut, within the caudal flexure (Fig. 977). Between 

 these is the wide opening of the yolk-sac, which is gradually narrowed and reduced 

 to a small foramen leading into the vitelline duct. At first the fore-gut and hind- 

 gut end blindly. The anterior end of the fore-gut is separated from the stomo- 

 deum by the buccopharyngeal membrane (Fig. 977); the hind-gut ends in the 

 cloaca, which is closed by the cloacal membrane. 



Thalamencephalon 



Optic vesicle 



Mid-brain 



Buccopharyngeal / 

 membrane * 



Pharynx 

 Auditory pit 



Bulbus cordis 



Stomach 



Cloaca' 



Body-stalk 

 Umbilical vein'' 



Allantois 

 Umbilical artery 



FIG. 977. Human embryo about fifteen days old. Brain and heart represented from right side. Digestive tube and 



yolk sac in median section. (After His.) 



The Mouth. The mouth is developed partly from the stomodeum, and partly 

 from the floor of the anterior portion of the fore-gut. By the growth of the head 

 end of the embryo, and the formation of the cephalic flexure, the pericardial area 

 and the buccopharyngeal membrane come to lie on the ventral surface of the 

 embryo. With the further expansion of the brain, and the forward bulging of the 

 pericardium, the buccopharyngeal membrane is depressed between these two 

 prominences. This depression constitutes the stomodeum (Fig. 977). It is lined 

 by ectoderm, and is separated from the anterior end of the fore-gut by the bucco- 

 pharyngeal membrane. This membrane is devoid of mesoderm, being formed 

 by the apposition of the stomodeal ectoderm with the fore-gut entoderm; at the 

 end of the third week it disappears, and thus a communication is established 

 between the mouth and the future pharynx. No trace of the membrane is found 

 in the adult ; and the communication just mentioned must not be confused with the 

 permanent isthmus faucium. The lips, teeth, and gums are formed from the walls 

 of the stomodeum, but the tongue is developed in the floor of the pharynx. 



The visceral arches extend in a ventral direction between the stomodeum and 

 the pericardium; and with the completion of the mandibular arch and the formation 

 of the maxillary processes, the mouth assumes the appearance of a pentagonal 

 orifice. The orifice is bounded in front by the fronto-nasal process, behind by the 



