GEKMINATING ENDOTHELIUM. 9 



perfectly spread out ; the glass slide is raised out of the fluid, and the 

 preparation is covered with a cover glass. 



Equally important are preparations stained with nitrate of silver 

 solution ; the membrane is excised, together with the organs to which 

 it is attached, e.g. intestine for the mesentery, sternum and peri- 

 cardium for the pleura mediastini, costal muscles with their corre- 

 sponding ribs for the centrum tendmeum, stomach spleen and pancreas 

 for the omentum of guinea-pigs and rabbits (for the omentum of the 

 cat, dog, and monkey, this is unnecessary). The excised organs are 

 placed in a large quantity of quarter or half per cent, solution of 

 nitrate of silver for a few minutes, and then treated in the ordinary 

 way. In mounting the preparations in glycerine the same precau- 

 tions are necessary as in mounting the fresh preparations, as above 

 mentioned, viz. those portions which are to be mounted are spread 

 out on the glass slide under water, and covered afterwards with a cover 

 glass, on the under surface of which a drop of glycerine has been pre- 

 viously applied. A good plan is to pour over the membranes diluted 

 serum before staining with silver. The risk of mechanical injury is best 

 avoided by pouring the silver solution on the membranes in situ, and 

 then cutting them out with their corresponding attachments, so as to 

 transfer them to water. We may mention at present that in our 

 farther investigations, especially in those of the cellular structures of 

 the tissue and of the lymphatics, we have always found that the treat- 

 ment of the serous membranes requires very great precautions against 

 even slight mechanical injuries, for only then can the appearances of 

 irregularities be avoided, which, especially in silver stained prepara- 

 tions, are apt to confuse the observer. In the contrary cases, there 

 may be found appearances of not pre-existing structures, even with 

 some regularity. 



It may not be unnecessary to make some remarks about the dark 

 lines which appear on silver-stained preparations, as the boundaries 

 of the endothelial cells. Some authors have tried to show that these 

 lines correspond to a precipitation in a serous fluid which is accu- 

 mulated in furrows on the surface of the endothelium, the individual 

 endothelial cells being somewhat convex on their upper surface. 

 There will scarcely be anyone who will not admit that (1) there 



