THE EPITHELIAL TISSUES. 



47 



tively more pervious than elsewhere. They are called pseudostomata, 

 to distinguish them from the stomata already mentioned. 



FIG. 24. 



Endothelium on a serous surface of the frog. (Klein.) a, stoma bounded by endothelial cells 

 with granular cytoplasm; b, pseudostoma. The nuclei of the cells are not represented. 



The intercellular substance in endothelium is so small in amount 

 and so homogeneous and transparent that it escapes observation 



FIG. 25. 



Endothelial lining of a small vein treated with silver nitrate ; dog. (Engelmann.) The fig- 

 ure represents a tube formed of endothelium the cells of which vary in size and shape. 

 The whole wall of a capillary has essentially the same structure as this venous lining ( 

 but its calibre is smaller. The upper branch in this figure may represent a capillary 

 opening into the vein, a, a, pseudostomata occupied by cement-substance. 



under the microscope unless special means are employed for its dem- 

 onstration. The simplest of these consists in treating the fresh 



