160 



ALIMENTARY CANAL 



In front of the sulcus terminalis the circumvallate papillae begin to show about the beginning 

 the third month (Graberg). Their position is marked by a pair of ridges diverging in front, but 

 meeting in the middle line behind. On these ridges circular epithelial thickenings appear whi( 

 grow inwards and cut, as it were, the papilla out of the mucous membrane. A fissure 

 appears in the ingrowing wall of epithelium, produced by the shedding of the central cells, whic 

 becomes the trench round the papilla, while the marginal thickening on the surface into which tl 

 stratum proprium extends becomes the vallum. The fungiform papillae appear about the begii 

 ning of the third month and the filiform a trifle later, both as projections of the connects 

 tissue, over which the epithelium is thickened. The epithelial plaque in the case of the filifori 

 papillae is afterwards broken up by the irregular thickening of the epithelium over them. 



The visceral pouches of the pharynx begin to disappear towards the end of the 

 first month. The dorsal portions of the first pair become the tubo-tympank 



FIG. 203. PEOFILE SKETCHES OF TWO STAGES IN THE DEVELOPMENT OF THE ALIMENTABY CANAL ix 



THE HUMAN EMBRYO. (His.) 



Ch., notochord; Sd (in A), median rudiment of thyroid; U.K., section of mandibular arch; 

 .R.T., hypophysis; Lg., lung ; K., larynx : Mg., stomach; P., pancreas ; Lbg., bile-duct ; Ds., vitelline 

 duct ; Zg. (in B), tongue ; AIL, allantois ; B., entodermic cloaca ; W., Wolffian duct ; N., hind kidney. 



passages (see Development of the Ear), the second in part form the pockets in 

 which the tonsils develop (Hammar). No traces of the remaining pouches persist 

 in the adult, but in the embryo their epithelial lining gives origin to important 

 organs. Thus the third pouches give origin to the thymus, and the fourth to the 

 lateral portions of the thyroid gland, while both third and fourth supply epithelial 

 buds which form the parathyroid bodies. 



(ESOPHAGUS, STOMACH, AND INTESTINES. 



Immediately behind the pharynx, the fore-gut contracts again to form the 

 O3sophagus, which is very short (fig. 203, A) in the early embryo, owing to the 

 imperfect development of the neck. Behind the oesophagus the gut widens out into 

 the dilatation which represents the stomach. This organ, which is at first nearly 



