DEVELOPMENT. 



7oa 



tympanum, and the external meatus. The mucous membrane of the 

 mouth, which is prolonged in the form of a diverticulum through the 

 Eustachian tube into the tympanum, and the external cutanous system, 

 come into relation with each other at this point ; the two membranes 

 being separated only by the proper membrane of the tympanum. 



The pinna or external ear is developed from a process of integument 

 in the neighborhood of the first and second visceral arches, and probably 

 corresponds to the gill-cover (operculum) in fishes. 



The Nose. The nose originates, like the eye and ear, in a depres- 

 sion 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, gradually extends backwards and downwards 

 till it opens into the mouth. 



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

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

 closed canal. 



THE ALIMENTARY CANAL. 



The alimentary canal in the earliest stages of its development con- 

 sists of three distinct parts the fore and hind gut ending blindly at. 



FIG. 489. Outlines of the form and position of the alimentary canal in successive stages of its 

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

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

 duodenum; i, the small intestine ; i\ the large; e, 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; 

 v i, the vitello-intestinal duct; w, the urinary bladder and urachus leading to the allantois; g, geni- 

 tal ducts. (Allen Thomson.) 



each end of the body, and a middle segment which communicates freely 

 on its ventral surface with the cavity of the yelk-sac through the vitel-- 

 line or omphalo-mesenteric duct. 



