THE HISTOGENESIS OF DENSE LYMPHATIC TISSUE 179 



epithelium to be secondarily acquired, due to the rapid growth 

 of the lymphatic tissue. Jolly ('11), who noted the extraordinary 

 infiltration of the epithelium over the intestinal tonsils, included 

 them (i.e., the infiltrated epithelium) with the tonsils of the 

 mouth and the thymus in one group, the ' lympho-epithelial 

 organs, ' in which he believes there exists a symbiotic relationship 

 between the epithelial cells and the lymphocytes. 



The lymphocytes, as stated above, first began to invade the 

 epithelium to any extent about fourteen days after birth. Of 

 course, from the very first appearance of lymphocytes in the 

 tunica propria, some may be found in the epithelium but they 

 do not invade it in great numbers till an age of two weeks or more 

 has been reached. Until that time the basement membrane of the 

 epithelium is quite complete, making a definite boundary be- 

 tween the epithelium and the underlying tissue, clearly indicating 

 the impossibility of any of the elements of the lymphatic tissue 

 taking origin from the cells of the epithelium. As more and 

 more lymphocytes crowd into the epithelium the basement mem- 

 brane becomes gradually less distinct, and the epithelium, 

 at first was of a simple columnar type, gradually acquires the 

 appearances of a heavily infiltrated, stratified epithelium, be- 

 cause of the displacement of the epithelial cells by the invading 

 lymphocytes. Its thickness from cuticular border to basement 

 membrane is greatly increased, but it really resembles an epithe- 

 lium very little because of the huge number of contained lympho- 

 cytes. The invading cells, for unknown reasons, are all of the 

 small lymphocyte variety. About each lymphocyte soon after 

 it enters the epithelium is found a clear area, indicating some 

 chemical activity by the lymphocytes upon the epithelial cells 

 immediately surrounding them (fig. 12). 



Not all of the lymphocytes found in the epithelium have 

 wandered in from the underlying tissue, as there is evidence of 

 proliferation of those which have already invaded it (fig. 12). 

 Mitotic figures are occasionally seen in lymphocytes in this 

 location. The spaces among the epithelial cells in which the 

 lymphocytes rest are seen to often contain in later stages several 

 small lymphocytes which have doubtless been derived by succes- 



