THE DIGESTIVE SYSTEM 303 



In the intestines a proliferation of the epithelium and of the underlying 

 stroma results in the formation of villi. These appear about the tenth week, 

 in both small and large intestines. In the former they increase in size, while 

 in the latter they atrophy and ultimately disappear. The simple tubular 

 glands of the intestines develop in a manner similar to those of the stomach. 



The mesothelium of the serosa is derived from the mesodermic cells of the 

 primitive body cavity. 



The development of the larger glands, connected with the digestive tracts 

 takes place in a manner similar to the formation of the simple tubular glands. 

 All originate in extensions downward of cntodcrmic cords into the underlying 

 mesodermic tissue. From the lower ends of these cords, branches extend in 

 all directions to form the complex systems of tubules found in the compound" 

 glands. 



The salivary glands being developed from the oral cavity, originate in similar 

 invaginations of ectodermic tissue. 



The pancreas originates as three separate evaginations from the entoderm of 

 the future duodenum. Of these, one atrophies, while the other two unite to 

 develop the adult pancreas. The two evaginations account for the two 

 pancreatic ducts. The evaginations take place into the underlying mesenchyme, 

 which develops the connective-tissue framework of the organ. The growths are 

 at first solid cords of cells which later become hollowed out to form tubules and 

 differentiate chief and centro-acinar cells, the former early showing zymogen 

 granules. Increase in size of the gland is accomplished by continuous budding 

 and extension of tubules. In some of the tubules the epithelial cells become 

 darker and arranged as a wall around a mass of developing blood cells. These 

 structures are known as primary islands. Neither their function nor subsequent 

 history is known. Later in development some of the gland tubules lose their 

 lumina, the zymogen granules disappear from their cells and the whole becomes 

 transformed into a secondary island or island of Langerhans of the adult pancreas. 

 According to Lageusse a transformation of islands into tubules also occurs, and 

 this process of transformation of tubules into islands and islands into tubules 

 persists throughout life. Claude on'- the other hand derives both primitive 

 islands and islands of Langerhans from the mesenchyme. Recent observations 

 tend to support the former of these views. 



The liver originates as a downgrowth of the entoderm of the ventral wall 

 of the future duodenum into the mesoderm of the transverse septum. At its 

 cephalic end the downgrowth is solid and gives rise to the liver; at the caudal 

 end it is hollow and gives rise to the gall-bladder. As the evagination increases 

 in size it becomes almost completely separated from the intestine, the slender 

 connection which does remain becoming the ductus choledochus. Between the 

 latter and the liver anlage a slender connection also remains, the cystic duct. 

 The mesenchyme surrounding these structures develops their connective-tissue 

 framework. As the liver outgrow'ths develop they come into relation with the 

 omphalomesenteric veins in such a manner that the vessels are broken up into 

 an anastomosing network of smaller vessels, while the liver cells develop into- 

 anastomosing hepatic cylinders. These resemble the tubules of other glands 

 in that the walls are formed by many cells. The manner in which these are trans- 



