94 



THE TISSUES. 



openings between the mesothelials cells through which fluids and 

 solid particles may pass into underlying lymph spaces. They are, 

 however, now generally regarded as artifacts. 



Endothelial cells are differentiated mesenchymal cells. They line 

 the blood- and lymph-vessels and lymph-spaces (arachnoidal and 



Fig. 58. Mesothelium covering posterior abdominal wall of frog. Stained with silver 

 nitrate and hematoxylin. 



Fig- 59. Endothelial cells from small artery of the mesentery of a rabbit. Stained with 

 silver nitrate and hematoxylin. 



synovial spaces, anterior chamber of the eye, bursae, and tendon 

 sheaths). Endothelial cells are in structure like those of the meso- 

 thelium. In blood- and lymph-vessels they are of irregular, oblong 

 shape, with serrated borders. The boundaries of these cells are 

 clearly brought out by silver nitrate. 



TECHNIC 



Epithelium may be examined in a fresh condition. The sim- 

 plest method consists in placing some saliva under a cover-glass and 

 examining it with a moderate power. In it will be found a number of 

 isolated squamous epithelial cells, suspended in the saliva singly and in 

 groups. The cells that are cornified still show the nucleus and a small 

 granular area of protoplasm. 



