THE STRUCTURE OF THE ELEMENTARY TISSUES. 31 



is placed in another dye, such as haematoxylin, which has the property 

 of selecting and staining its nuclei. 



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

 the mesentery of the frog, especially during the breeding season. 



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. 



Besides the ordinary endothelial cells above described, there are 

 found on the omentum and parts of the pleura of many animals, little 

 bud-like processes or nodules, consisting of small polyhedral granular 

 cells, rounded on their free surface, which have multiplied very rapidly 

 by division (figs. 22 and 23). These constitute what is known as ger- 

 minating endotlielium. The process of germination doubtless goes on 

 in health, and the small cells which are thrown off in succession are 



Fig. 22. Peritoneal surface of a portion of the septum of the great lymph-sacs of frog. The 

 stomata, some of which are open, some collapsed, are surrounded by endothelial cells. (Klein.) 



carried into the lymphatics and contribute to the number of the lymph 

 corpuscles. The buds may be enormously increased both in number 

 and size in certain diseased conditions. 



On those portions of the peritoneum and other serous membranes in 

 which lymphatics abound apertures (fig. 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 



