THE PKIMITIVE ALIMENTAKY CANAL. 



45 



it has the form of a longitudinal groove bounded at its cranial end and laterally by 

 an elevated ridge, named' by His the furcula (Fig. 59). The caudal end of the groove 

 soon dilates into a pouch, and then the pouch and groove are separated by a con- 

 striction, which passes from the caudal 

 towards the cranial end, from the more 

 dorsal part of the fore -gut, which be- 

 comes the oesophagus. The constricting 

 process ceases before the separation 

 reaches the cranial extremity of the re- 

 spiratory rudiment, which remains, there- 

 fore, in communication with the pharynx 

 and forms the permanent laryngeal aper- 

 ture. The tube formed by the separation 

 of the groove is differentiated into the 

 larynx and the trachea, and the caudal 



terminal dilatation SOOn divides into two FlG - 59. VIEW OF FLOOR OF PKIMITIVE PHARYNX, 

 u 1^'^u n, 4-Vm ..T,rJ4 showing the furcula with the groove, from which 



lateral lobes, each of which is the rudi- arise t | e cavities of the laryn * the ' trachea) the 

 ment of the epithelial lining bronchi and bronchi, and the alveoli of the lungs, 

 the lung of the corresponding side. 



The Tongue. The tongue is formed by four separate rudiments which lie in the 

 ventral part of the cranial end of the primitive pharynx. Two of these are eleva- 

 tions formed on the caudal surfaces of the ventral ends of the mandibular arches, 



Tuberculum impar 

 Sinus arcuatus 



Furcula 



Ccelon: 



Pancreas 

 Stomach 

 Bile-duct 



Small intestine 



Caecum 



Liver 



Lung 



Intestinal loop 



Large intestine 



.rNotochord 



(Esophagus 



Trachea X. 



Vertebra 



Heart 



Wolffian duct 



Kidney 



Mouth 



~ Proctod^um 



Hind-brain ^^^^^^ ^^^^^^ 



Allantoic diverticulum 



Mid-brain | Vitello-intestinal duct 



Fore-brain 



FIG. 60. FURTHER DEVELOPMENT OF THE ALIMENTARY CANAL, AS SEEN IN A HUMAN EMBRYO 

 ABOUT FIVE WEEKS OLD (Diagrammatic). 



The tongue is well formed, the trachea and oesophagus are separated, the bronchi have commenced to branch ; 

 the duodenal curve is well formed, and the caecum has appeared in the loop of the mid-gut. The 

 cloaca is partially separated into genito-urinary and rectal portions. 



one on each side. The third is a median elevation, the tuberculum impar, which is 

 situated immediately caudal to the conjoined ventral ends of the mandibular arches, 



