GENERATION AND DEVELOPMENT. 



295 



Nose. The nose originates like the eye and ear in a depression of 

 the superficial epiblast at each side of the fronto-nasal process (primary 

 olfactory groove), which is at first completely separated from the cavity 

 of the mouth, and gradually extends backward and downward till it 

 opens into the mouth. 



The outer angles of the fronto-nasal process, uniting with the maxil- 

 lary process on each side, convert what was at first a groove into a closed 

 canal. 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



The alimentary canal in the earliest stages of its development consists 

 of three distinct parts the fore and hind gut ending blindly at each end 

 of the body, and a middle segment which communicates freely on its 



FIG. 454. Outlines of the form and position of the alimentary canal in successive stages of its 

 development. A, alimentary canal, etc., in an embryo of f9ur weeks; B, at six weeks; C, at eight 

 weeks; D, at ten weeks; Z, the primitive lungs connected with the pharynx; , the stomach; d, the 

 duodenum; i, the small intestine; i', the large; c, the caecum and vermiform appendage; r, the rec- 

 tum; cl, in A, the cloaca; a, in B, the anus distinct from s i, the sinus uro-genitalis; v, the yelk-sac; 

 vi, the vitello-intestinal duct; it, the urinary bladder and urachus leading to the allantois; <jr, genital 

 ducts. (Allen Thomson . ) 



ventral surface with the cavity of the yelk- sac through the vitelline or 

 omphalo-mesenteric duct (p. 261, Vol. II.). 



From the fore-gut are formed the pharynx, oesophagus, and stomach; 

 from the hind-gut, the lower end of the colon and the rectum. The 

 mouth is developed by an involution of the epiblast between the maxillary 

 and mandibular processes, which becomes deeper and deeper till it reaches 

 the blind end of the fore-gut, and at length communicates freely with 

 the pharynx by the absorption of the partition between the two. 



At the other end of the alimentary canal the anus is formed in a 



