C. ROUSSELET ON A METHOD OF PRESEEVING ROTATORIA. 207 



at the same time. It is essential that the animals should not 

 be quite dead when the killing and fixing solution is added. 

 As soon as a Rotifer is quite dead various post-mortem changes 

 begin immediately to take place in the tissues, first absorption 

 of water and swelling, then disintegration and decomposition, 

 and it is evident that in order to preserve the animal in its 

 natural state, it is necessary to fix the histological elements 

 before any such changes have taken place. The word "fixing" 

 implies rapid killing and at the same time hardening of the 

 tissues to such an extent as to prevent their undergoing any 

 further change by subsequent treatment with preserving fluids. 

 The animals remain in the fixing solution a quarter to half an 

 hour, not longer, small Rotifers rather less ; then the solution 

 must be washed out with distilled water by changing the 

 water five or six times. The animals will then be ready to be 

 placed in the preserving fluid. 



Preserving. — The choice of a suitable preserving fluid has been 

 a matter of some difiSculty, and may possibly still be improved 

 upon. The required qualities are, that it should not alter the 

 form and tissues of the fixed Rotifers ; that it should not form 

 a deposit or crystals ; that it should not attack the cements used 

 for making and mounting in cells ; and that it should have a 

 density not differing appreciably from that of water. 



Alcohol and glycerine prevent decomposition by absorption 

 of water ; this means shrinkage in the delicate tissues of 

 Rotifers, both these fluids are, therefore, unsuitable. Weak 

 solution of corrosive sublimate, 2 in 1,000 parts, has several 

 times produced crystals, and has also attacked Miller's 

 caoutchouc cement, of which my cells are made, and from which 

 it appears to dissolve out some of its constituents. 



I have tried various other liquids with more or less success, 

 and have come to the conclusion that the best preserving fluid 

 is simply distilled water rendered antiseptic by a trace of the 

 fixing solution (about 8 drops of Flemming's solution in an 

 ounce of water), giving the slightest possible yellow tinge to 

 the water. Rotifers mounted in this solution for six months 

 have kept very well. The chromic acid appears to give a slight 

 yellowish colour to the tissues, but otherwise they remain very 

 fairly transparent ; it is possible, however, that with further 

 experience a still better preservative fluid will be found. 



