308 



HISTOLOGY 



sum. A small amount of the underlying lymphatic tissue appears, 

 and lymph cells are seen passing through in greater numbers in the right- 

 hand figure. They almost 



A //&&ti\ B . take away the character- 



istic appearance of the 

 epithelium at this point, 

 making it look like lym- 

 phatic tissue instead. A 

 thin, distal layer of strati- 

 fied cells persists in nearly 

 all stages, being renewed, 

 when the leucocytes have 

 destroyed it by passing 

 through in large num- 

 bers. 



There are many other 

 places in the digestive 

 tract in which lymphatic 

 tissue is put in this rela- 

 tion with the enteric 

 lumen. Sometimes a sim- 

 ple epithelium forms the 

 surface through which the 

 leucocytes break. Where 

 there is but little of the 

 adenoid tissue, there are 

 no lymph nodules to pro- 

 pagate a new supply of 

 the lymph cells, and these 

 are brought or wander in 

 from other germinal cen- 

 ters instead. Amoeboid cells resembling leucocytes are always to be 

 seen between the columnar cells which line the larger part of the 

 enteron. At other places the lymph tissue is collected locally into iso- 

 lated masses which, when large enough, are found to contain a germinal 

 center, and here the amoeboid cells break through the epithelium en 

 masse. Such are the Peyer's patches of the small intestine. 



Another pair of embryonic gill clefts, the third, invaginate as did the 

 tonsil glands to form another, and temporarily larger, gland which is 

 called the thymus gland. It is a much more specialized tissue than the 

 tonsil, and grows rapidly until a rather early period in life, after which 

 it undergoes a retrograde development, and finally degenerates in age 

 by a fatty transformation. Its structure and development are difficult 



FIG. 277. A and B, epithelium from two regions in the 

 tonsilar cleft of a young opossum, d.s., distal surface of 

 the stratified epithelium ; b.m., basal membrane of epithe- 

 lium; lu., leucocytes crawling through epithelium and in 

 lumen; lym.t., lymphatic tissue beneath the epithelium. 

 X 650. 



