200 Modern Microscopy 



2J per cent, formalin, the animals may be kept in little 

 bottles, or mounted in the same fluid on micro-slides, either 

 in excavated cells or shallow cement cells. 



Mounting. To mount on a slide, place a drop of the 

 iormalin solution in the cell, then transfer the prepared 

 rotifers into this drop with the pipette, and examine under 

 the dissecting microscope to see that no particle of foreign 

 matter has been introduced. Then place another drop of 

 the fluid on the slide by the side of the cell, lower the cleaned 

 cover-glass on that drop, and push the cover cautiously 

 and gradually over the cavity. The superabundant fluid 

 is removed with blotting-paper, and the slide closed by 

 tipping damar-gold size cement all round the edge with a 

 fine brush. 



The permanent closing of these cells has been a matter 

 of very considerable difficulty. As the result of the ex- 

 perience gained, it is recommended that the cells be closed 

 first with a coat of a varnish consisting of two-thirds damar 

 in benzole and one-third gold size, then two coats of pure 

 shellac dissolved in alcohol, and finally four to six coats of 

 pure gold size. Each layer of cement must be allowed to 

 dry thoroughly well ; three days for each layer is not 

 too long. 



By the method described above, Mr. Eousselet has in the 

 course of the last ten years made a collection of over 500 

 slides containing nearly 300 different species of rotifers, 

 probably the only collection of the kind in existence, which 

 is of the greatest use for the identification of species and for 

 the general study of this interesting class. 



Entomostraca should be narcotized with the same solu- 

 tion as used by Mr. Rousselet for Eotifera, then killed with 

 a J per cent, solution of osmic acid, and mounted in a 2 J per 

 cent, solution of formalin. 



