240 BULLETIN OF THE UNIVERSITY OP WISCONSIN. 



munication with the former bj means of circular openings — 

 stomata. 



Impregnation. This consists in treating the serous mem- 

 branes with some stain, generally nitrate of silver, which will 

 bring out the cell boundaries. Most of the later investigations 

 have been carried on by this method and it is the one I have 

 used exclusively. 



Facts obtained by the first two methods, and to some extent 

 by the last, have lead many authors to assert that the irregular 

 distributed figures found between endothelial cells are stomata. 

 That such figures occur can not be denied ; but are they neces- 

 sarily natural pre-formed openings ? Or, granting that stomata 

 are present in the frog, does it necessarily follow that they are 

 present in mammals, which present a higher and more complex 

 structure ? 



Significance of Stomata. Here again, as on the whole ques- 

 tion of stomata, the greatest diversity of opinions exists. The 

 greater number of the investigators who have expressed any 

 opinion in this matter, regard stomata as free openings afford- 

 ing passage to the superficial lymphatics. Ludwig, Schweigger- 

 Seidel, Klein, Rawitz, and others, speak of the passage of leuco- 

 cytes through these openings, and Ellenberger (5) adds that they 

 are sometimes plugged up with lymph cells. After referring 

 to the work of Schweigger-Seidel and Ranvier, Ellenberger says : 

 ''These (the gaps in the intercellular substance) lead directly 

 into the lymph-spaces." 



Ranvier (17), however, did not hold this opinion, for he says: 

 "Most hisfologists consider the small inter-endothellal spots as pre- 

 existing stomata designed for the free passage of the lymph cells; 

 * * * but the irregular distribution of these spots and their absence 

 if, previous to the employment of silver nitrate, the almuminate cov- 

 ering the surface has been removed, argues against the presence of 

 pre-existing openings. The presence of these spots upon the omen- 

 tum which is already so largely perforated, and where the passage 

 of white corpuscles is perfectly unnecessary, adds a further argument 

 against the opinion of those who would call these structures stomata." 



In his chapter on the structure of the capillaries Ranvier ex- 

 presses himself as strongly opposed to the presence of the 



