DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 323 



The early entodermal cells proliferate, and the resulting cells change ac- 

 cording to their position in the gland. Those lining the larger ducts become 

 high columnar, with more or less homogeneous cytoplasm; those lining the 

 intermediate (intercalated) ducts become low; those lining the terminal secret- 

 ing tubules become pyramidal and more highly specialized, and also acquire 

 certain constituents the zymogen granules (Fig. 281, c) which vary with the 

 functional activities of the gland. The centro-tubular cells in the terminal 

 tubules are probably to be explained on a developmental basis. While a few 

 maintain that they are "wandering" cells, it is quite generally accepted 

 that they are simply continuations of the flat cells lining the intermediate 

 ducts, the result being that the cells of the terminal tubules seem to 

 spread out over the ends of the intermediate ducts in the form of cap-like 

 structures. 



It was once thought that the islands of Langerhans were derived from the 

 mesodermal tissue. Recently it has been pretty clearly demonstrated that they 

 are derived from entoderm. In guinea-pig embryos of 5 to 6 mm., at a time 

 when the dorsal pancreas has merely begun its constriction from the gut, certain 

 cells in the mass appear darker and slightly larger than the others. They show 

 darker areas of cytoplasm around the nuclei, and later the darker areas extend 

 throughout the cells and the nuclei become larger and more vesicular. When 

 lumina appear in the outgrowths or buds, these cells occupy a position on or near 

 the surface of the buds (Fig. 281, a). In further development they tend to sepa- 

 rate themselves from the buds and collect in clumps (Fig. 281, b). Capillaries 

 then penetrate the clumps and break them up into the trabeculae of cells char- 

 acteristic of the islands of Langerhans (Fig. 281, c). Studies on the development 

 of the islands in the human pancreas indicate a similar origin and mode of 

 development. 



Anomalies. 



One of the most striking anomalies of the organs of alimentation is found 

 in connection with a more general anomalous condition known as transposition 

 of the viscera (situs viscerum inversus) . The transposition may be so complete 

 that the minor asymmetries normally present on the two sides are all repeated 

 in reverse order, the functions of the organs being unimpaired. As regards the 

 alimentary tract, this means that the position of the stomach is reversed in the 

 abdominal cavity; that the duodenum crosses from left to right; that the various 

 coils of the jejunum and ileum occupy positions opposite to the normal; that the 

 caecum and ascending colon are situated on the left side and the descending 

 colon on the right; and that the larger lobe of the liver lies on the left side. The 

 other visceral organs are transposed accordingly, the heart being inclined to- 

 ward the right side, the left lung consisting of three lobes and the right of two, 



