ENDOTHELIUM 



61 



structure, and differs but little from the arrangement on human 

 serous surfaces. 



Kill a large frog by decapitation, and open the abdomen freely 

 by an incision along the median line. Pull out the intestines 

 grasping the stomach with the forceps. This will expose the small 

 intestine, which you will remove, together with the attached mes- 

 entery, by means of quick snips of the scissors. Work as rapidly 

 as possible and avoid soiling the tissue with blood. Throw the 

 gut into a saltcellar filled with silver solution (vide formula), 

 where it must remain for ten minutes covered from the light. Lift 

 the tissue from the solution by means of a strip of glass (or a 

 platinum wi#e), and throw into a saucer of clean (preferably dis- 

 tilled) water, changing the latter repeatedly for some minutes. 



FIG. 36. ENDOTHELIUM PROM FROG'S MESENTERY. SILVER STAINING. 



A. Area showing the outlining of the cells by the silver stained cement-substance. The 

 nuclei have been brought out by the carmine. Minute stomata may be seen between certain 

 cells. 



B. A blood-capillary terminating below in an arteriole. The silver has outlined the endo- 

 thelial cells of the vessels. 



C. An area showing both layers of the cells. The deeper cells are faintly outlined, being 

 out of focus. The silver has been deposited over the lower portion of the specimen, nearly 

 obscuring the cement lines (X 250). 



