THE DIGESTIVE TRACT. 



eral increase in the thickness of the young mucosa these structures 

 lengthen and obtain their lumen. The lower ends of the glands 

 throughout the period of their growth are the seats of active cell 

 proliferation and the points at which the division of their fundi com- 

 mences in the production of the compound tubules. The endothe- 

 lium covering the serous surfaces of the intestinal tract is the direct 

 descendant of the differentiated mesoderm, the mesothelium, lining 

 the body-cavity. 



The development of the accessory glands of the digestive 

 tube, including the liver, the pancreas, and the salivary glands, 

 follows the same general plan. The epithelial covering of the 

 primitive mucous membrane sends cylindrical masses of entodermic 

 or ectodermic elements, as the case may be, into the surrounding 

 mesoderm ; the originally single cord of cells very soon undergoes 

 division, a richly-branched system of epithelial tubes early represent- 

 ing the future gland. The liver originates as a ventral outgrowth 

 of the intestinal epithelium into the septum transversum ; very soon 

 this branches, the two hepatic diverticula following so closely 

 upon the stage of the single outgrowth that the latter is sometimes 

 overlooked. The walls of the distal ends of the diverticula soon 

 become greatly thickened, which areas of entodermic epithelium 

 represent the earliest traces of the hepatic tissue. Regarding the 

 details of the further stages in the growth of the more complicated 

 livers opinions do not agree ; it is probable, however, that the 

 hepatic cords of the mammalian organ are attributable to the same 

 general plan of development as are other tubular glands, the com- 

 plicated arrangement of the secreting tissue resulting from incomplete 

 separation and subsequent fusion of the cell-cords. The invasion of 

 the epithelial areas by the blood-vessels breaks up the entodermic 

 tissue into the cell-nests which occupy the intercapillary spaces. 

 Two forms of liver- cells are present during the greater part of 

 fcetal life, large polyhedral elements, and small round cells, the latter 

 disappearing shortly after birth ; the relation between the two varieties 

 is not clearly established, but the small cells are probably younger 

 stages of the larger. Multinucleated cells of considerable size 

 also occur, within the blood-vessels of the embryonal liver; these are 

 probably connected with the development of red blood-corpuscles 

 before birth. The lining of the bile-vessels and of the interlobular 

 bile-ducts, together with the hepatic cells, is a derivative from the 

 entoderm, while the connective tissue and blood-vessels, as well as 

 the tissues of the walls of the bile-vessels other than the epithelial 

 lining, are contributions from the mesoderm. 



The pancreas appears as three diverticula, one dorsal and two 

 ventral, which extend from the duodenum into the primitive mesen- 



