STRUCTURE OF THE ELEMENTARY TISSUES. 23 



and peripheral ganglion-cells. The cells are scaly in form, and irregular 

 in outline; those lining the interior of blood-vessels and lymphatics hav- 

 ing a spindle-shape with a very wavy outline. They enclose a clear, oval 

 nucleus, which, when the cell is viewed in profile, is seen to project from 

 its surface. 



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

 frogs, especially about the breeding season. 



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, round- 

 ed on their free surface, which multiply very rapidly by division (Fig. 

 17). These constitute what is known as "germinating endothelium." 



FIG. 1 i . Suver-stained preparation of great omentum of dog, which shows, amongst the flat 

 endothehum of the surface, small and large groups of germinating endothelium, between which 

 numbers of stomata are to be seen. (Klein.) x 300. 



The process of germination doubtless goes on in health, and the small cells 

 which are thrown off in succession are 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 condi- 

 tions. 



On those portions of the peritoneum and other serous membranes 

 where lymphatics abound, there are numerous small orifices stomata 

 (Fig. 18) between the endothelial cells: these are really the open mouths 

 of lymphatic vessels, and through them lymph -corpuscles, and the serous 

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



2. Split roiihil epithelial cells are the active secreting agents in most 



