26 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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- 



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. 



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

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



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 



FIG. 22. Peritoneal Surface of a Portion of the Septum of the great Lymph Sac of Frog. 

 The stomata, some of which are open, some collapsed, are surrounded by endothelial cells 

 (Klein.) X 160. 



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. 



Simple Non-ciliated Columnar Epithelium, figure 23, lines, a, the mucous 

 membrane of the stomach and intestines as a single layer, from the cardiac 



