650 DEVELOPMENT OF THE ALIMENTARY CANAL 



lish a communication with the intestinal tube. The formation of 

 the anterior perforation, and its appendages, takes place in the fol- 

 lowing manner : 



After the early development of the intestinal tube in the mode 

 above described, the head increases in size out of all proportion to 

 the remainder of the foetus, projecting as a large rounded mass from 

 the anterior extremity of the body, and containing the brain and the 

 organs of special sense. This portion soon bends over toward the 

 abdomen, in consequence of the increasing curvature of the whole 

 body which takes place at this time. In the interior of this 

 cephalic mass there is now formed a large cavity (Fig. 241, d), by 

 the melting down and liquefaction of a portion of its substance. 

 This cavity is the pharynx. It corresponds by its anterior extre- 

 mity to the future situation of the mouth ; and by its posterior 

 portion to the upper end of the intestinal canal, the future situation 

 of the stomach. It is still, however, closed on all sides, and does 

 not as yet communicate either with the exterior or with the cavity 

 of the stomach. There is, accordingly, at this time, no thorax 

 whatever ; but the stomach lies at the upper extremity of the abdo- 

 men, immediately beneath the lower extremity of the pharynx, from 

 which it is separated by a wall of intervening tissue. 



Subsequently, a perforation takes place between the adjacent 

 extremities of the pharynx and stomach, by a short narrow tube, 

 the situation of which is marked by the dotted lines x, in Fig. 241. 

 This tube afterward lengthens by the rapid growth of that portion 

 of the body in which it is contained, and becomes the cesophagm. 

 Neither the pharynx nor oesophagus, therefore, are, properly speak- 

 ing, parts of the intestinal canal, formed from the internal layer of 

 the blastodermic membrane ; but are, on the contrary, formations 

 of the external layer, from which the entire cephalic mass is pro- 

 duced. The lining membrane of the pharynx and oesophagus is to 

 be regarded, also, for the same reason, as rather a continuation of 

 the integument than of the intestinal mucous membrane ; and even 

 in the adult, the thick, whitish, and opaque pavement epithelium 

 of the oesophagus may be seen to terminate abruptly, by a well- 

 defined line of demarkation, at the cardiac orifice of the stomach ; 

 beyond which, throughout the remainder of the alimentary canal, 

 the epithelium is of the columnar variety, and easily distinguish- 

 able by its soft, ruddy, and transparent appearance. 



As the oesophagus lengthens, the lungs are developed on each 

 side of it by a protrusion from the pharynx which extends and 



