198 Modern Microscopy 



of the narcotic varies very much with different species ; 

 some are most sensitive to it, whilst others can stand a 

 considerable quantity for a long time. 



Killing and Fixing. — Some practice and patience are 

 certainly required to find out the right time to kill the 

 different species ; no general rule can be given, as the time 

 may vary from fifteen minutes to several hours. It is 

 very essential, however, that the rotifers be still living 

 when the killing fluid is added to prevent post-mortem 

 changes in the tissues which begin at once on the death of 

 the animals. 



For killing and fixing several fluids are suitable — namely, 

 J per cent, osmic acid, or Plemming's chromo-aceto-osmic 

 fluid, or Hermann's platino-aeeto-osmic mixture. On the 

 whole, I now prefer the last-named, which gives a finer 

 fixation of the cellular elements of the tissues and does not 

 stain them so much. It may be explained that the term 

 ' fixing ' implies rapid killing and at the same time harden- 

 ing of the tissues to such an extent as to render them 

 unalterable by washing and subsequent treatment with 

 preserving fluids. Proper fixation is very essential, as no 

 good preservation can be obtained without it. 



When the rotifers are narcotized and ready for killing, 

 a single drop of one of the above fixatives is added, and 

 mixed with the water in the watch-glass. A few minutes 

 is sufficient for fixing small creatures like these, and then 

 they must be removed again by means of the pipette to 

 several changes of clean water to get rid of the acid, 

 otherwise they will become more or less blackened. When 

 dealing with marine rotifers, sea water must be used for 

 washing out, for the difference in density between fresh and 

 sea water is sufficient to cause swelling by osmosis, and the 

 consequent spoiling of the specimen. After thoroughly 

 washing, the rotifers are transferred to a preserving fluid, 

 the density of which does not materially differ from that 

 of water. The best preserving fluid found so far is a 

 2J per cent, solution of formalin, which is made by mixing 



