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MANUAL OF PHYSIOLOGY. 



tois, which arises as a bud from the lower part of the rudimentary 

 hind gut. 



Between these two canals an intermediate one is formed by the 

 splanchnopleure, which, at a distance from its origin, becomes 

 constricted, and shuts off an upper canal, the mid gut, from a 

 lower larger organ, the yelk sack, the connection between the two 

 forming the duct as vitello-intestinalis. 



Thus the primitive alimentary canal consists of an anterior and 

 a posterior blind canal, which are closed below, and a canal in- 



FIG. 272. 



Alimentary canal of an embryo while the rudimentary mid gut is still in continuity 

 with the yelk sack. (KOlliker, after Bischoff.) A. View from below: a. Pharyngeal 

 plates, b. The pharynx, c.c. Diverticula forming the lungs, d. The stomach. /. Di- 

 verticula of the liver, g. Membrane torn from the yelk sack. h. Hind gut . B. Longi- 

 tudinal section : a. Diverticulum of a lung . 6. Stomach, c. Liver, d. Yelk sack. 



termediate between these, which opens at its lower surface into 

 the yelk sack. 



As the placental circulation becomes more and more developed, 

 so the yelk sack shrinks and atrophies, until at last it is represented 

 by a fold of tissue connected with the primitive intestine. The 

 ductus vitello-intestinalis accordingly becomes obliterated, and 

 thus the mid gut is closed at its lower aspect. 



The primitive intestine placed at the inferior aspect of the 



