238 THE CULTIVATION OF DIATOMS 



side of the filament that has been selected. Nevertheless, by this 

 first action, you have eliminated many species and have estab- 

 lished the predominance of the Melosiras in the preparation, 

 which, in contact with a new and specially suitable environment, 

 will attain in a few days a superb development, furnishing tresses 

 of filaments which rise in the liquid, and which allow you to 

 recommence the process, with the probability of insulating the 

 Melosiras absolutely. 



It is equally easy, with a little practice, to separate, under a 

 low-power microscope, Fragilarias, Diatomas, Biddulphias, etc., 

 grouped in longer or shorter chains, and to place them in the 

 nutritive liquids. But this operation is infinitely more difficult 

 when it is desired to separate one living Diatom. We may say, 

 without exaggeration, that every frustule seized by the tweezers is 

 a broken frustule and therefore incapable of reproduction. 



If the diatoms could be previously reduced to a dry state, the 

 difficulty that I have noted would be easily overcome. However 

 it may be, I attribute my failure solely to want of skill in with- 

 drawing from a maceration, either with a capillary tube, a bristle, 

 or the point of a forcep, any small diatoms previously determined 

 on. Success is a little more certain with the larger kinds, such as 

 the Coscinodisci and other species, that have a diameter or length 

 of a tenth of a millimetre. In this case they may be insulated on 

 the stage of the microscope, and by a bristle conducted into drops 

 of distilled water in series on the mounts, and finally on to a piece 

 of cover-glass placed in a new maceration. 



Observers who are in this way able to insulate with certainty 

 diatoms of all sizes will have no need to recur to the '' general " 

 processes, which require much longer manipulations. 



General Process for the Separation of Diatoms. 



The process that has hitherto given me the best results is that 

 which depends on the division — -fractionnemetit — of a previously 

 arranged culture. It consists in putting the diatoms in suspension 

 in such a volume of water that 5 ccm. of that water shall enclose 

 at least one frustule, which results in this, that when you sow 

 I ccm. of the dilution in five macerations, you will have four 

 sterile and one fruitful. This method requires a preliminary 

 experiment. 



