AN AQUATIC MYXOMYCETE, 73 



too little used in England, and it may be worth while to give a 

 description of the process, which can of course be made use of for the 

 cultivation of almost any minute organisms. 



A moist chamber is made by cutting a piece of thick cardboard, 

 or several thicknesses of filter paper, to the size of an ordinary glass 

 slide (3in. by lin.) and cutting a hole through the centre : this hole 

 may be about fin. in diameter, large enough to let all necessary light 

 pass through, but not too large to be efficiently covered by means of a 

 square or circular thin cover-slip, which will be held close down to the 

 perforated board or paper when the latter is saturated with distilled 

 water by the capillary attraction at the edges. The cardboard or 

 paper pad is then saturated with water, and can obviously be boiled 

 or heated if necessary, for a short time ; its wet surface adheres firmly 

 to the glass slip on which it is now placed, and which supplies a 

 transparent floor to the small cylindrical chamber, the walls of which 

 are formed by the saturated board or paper, and the roof by the thin 

 cover-slip. It is obvious that a small drop of water suspended from 

 the under side of the thin cover-slip, will evaporate very slowly, so 

 long as the cardboard or filter-paper is kept thoroughly wet, because 

 the air in the chamber will be nearly saturated with water ; hence 

 any spores &c. suspended in the drop will run no risk of drying up 

 for a considerable number of hours. Moreover, it is clear that light can 

 pass through the whole chamber, and its suspended drop, in sufficient 

 quantity to enable us to examine what is going on in the drop even 

 with fairly high powers, provided the cover-slip is sufficiently thin, 

 and the drop of water not so large that objects falling to the lower 

 surface of it are out of focus. At any rate it is easy to work with 

 Zeiss D and E in this way. Another great advantage to be claimed 

 for these damp cells is the ease with which atmospheric oxygen can 

 gain access to the interior : the comparatively trivial practical difficul- 

 ties need not be entered upon here. 



It is, of course, difficult to keep Bacteria, <fcc, out of such damp 

 cells — heated needles, pure water, treating all the glass parts with 

 acids, absolute alcohol or heat, &c, are all without avail unless the 

 water saturating the parts is kept clean, and, of course, unless spores 

 and foreign bodies are absent from the material sown, &c. Some 

 remarks on this subject will be made shortly, however, in connection 

 with certain experiments of physiological importance. 



