DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 345 



some dispute as to whether the mucous cells are continuously being derived 

 from the other epithelial cells or, when once formed, reproduce themselves by 

 mitosis. As a matter of fact, mitosis has been observed in the mucous cells of 

 the stomach. 



FIG. 308. From a reconstruction of the small intestine of a human embryo of 28 mm., showing the 

 longitudinal ridges which eventually become broken transversely to form the villi. Berry. 



THE LYMPH FOLLICLES. In the development of the lymph follicles in the 

 gastrointestinal tract the same question arises as in the case of the tonsils and 

 thymus. Are the lymphoid cells of mesodermal or of entodermal (epithelial) 



a 



'i \ 



'& 

 







FIG. 309. Sections through the wall of the caecum of (a) a rabbit 2^ days and (6) 5 days after 

 birth, showing the development of the lymph follicles. /, Lymphoid infiltration in the stroma; 

 r, wandering cells in the epithelium; z, lymphoid cells in the core of a villus. Stohr. 



origin? Evidence at present favors the mesodermal origin. In the case of 

 Peyer's patches, collections of lymphoid cells appear near the blood vessels in 

 the stroma and neighboring parts of the submucosa. These increase in extent, 



