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MANUAL OF PHYSIOLOGY. 



a short canal, the upper wall of which is formed by the general 

 blastoderm, and its lower wall by that part of the splauchnopleure 

 which runs backwards, leaving the somatopleure to form the 

 pleuro-peritoneal space. It then turns forwards to meet with 

 the uncleft mesoblast, forming the wall of the yolk-sac, which 

 communicates freely with this rudimentary part of the alimentary 

 tract. 



This canal becomes closed in for a considerable extent, and is 

 then called the/ore-^. It is the precursor of the pharynx, the 



FIG. 272. 



Alimentary canal of an embryo whilst the rudimentary mid-gut is still in 

 continuity with the yolk-sac. (Kolliker after Bischoff.) A. View from below : 

 a. Pharyngeal plates. 6. The pharynx, c.c. Diverticula forming the lungs. 

 d. The stomach. /. Diverticula of the liver, g. Membrane torn from the 

 yolk-sac, h. Hind-gut. B. Longitudinal section: a. Diverticulum of a 

 lung. 6. Stomach, c. Liver, d. Yolk-sac. 



lungs, the oesophagus, the stomach, and the duodenum. The 

 mouth, which at this period is unformed, is developed later by an 

 involution of the epiblast and the removal of the tissue between 

 the fore-gut and the buccal cavity. 



The tail fold, in a somewhat similar manner, shuts off a canal 

 called the hind-gut, which becomes developed into the posterior 



