20 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



is found arranged in a single layer of flattened cells, for example, the lining of 

 the alveoli of the lungs and of the descending arm of Henle's loop of the 

 kidney tubule. Aside from endothelium as mesothelium it has very limited 

 distribution in man. Endothelium and mesothelium are typical simple 

 .squamous epithelia. They consist of much flattened cells with clear or 

 .slightly granular protoplasm and oval bulging nuclei, the edges of the cells 

 .are peculiarly wavy or serrated. 



FIG. 20. The Endothelium of a Small Blood Vessel. Silver-nitrate stain. X 350. 



The presence of endothelium in any locality may be demonstrated by 

 staining with silver nitrate, which brings into view the intercellular cement 

 substance. When a small portion of a perfectly fresh serous membrane, 

 for example, figure 20, is immersed for a few minutes in a solution of silver 

 nitrate, and exposed to the action of light, the silver is precipitated in the in- 

 tercellular cement substance, and the endothelial cells are thus mapped out 

 by fine, dark, and generally sinuous lines of extreme delicacy. 



FIG. 21. Abdominal Surface of Central Tendon of the Diaphragm of Rabbit, showing the 

 general polygonal shape of the endothelial cells; each cell is nucleated. (Klein.) X 300. 



Endothelial cells in certain situations may be ciliated, e.g., those of the 

 mesentery of the frog, especially during the breeding season. 



On those portions of the peritoneum and other serous membranes in 

 which lymphatics abound, apertures, figure 22, are found surrounded by 

 small, more or less cubical, cells. These apertures are called stomata. They 

 are particularly well seen in the anterior wall of the great lymph sac of the 

 frog, figure 22, and in the omentum of the rabbit. These are really the open 

 mouths of lymphatic vessels or spaces, and through them lymph corpuscles 

 and the serous fluid from the serous cavity pass into the lymphatic system. 



