CONTIilBUTlONS TO THE CYTOLOGY OF THE CACTEKIA. 417 



distinguisliable from one another. The only real difference 

 observable is that the organisms which have been dried 

 appear slightly broader than those which have not — owing to 

 the slight flattening which drying brings about. The internal 

 structures appear exactly alike. Controls with wet films fixed 

 with sublimate-alcohol and stained witli Heidenhaiu^s irou- 

 hasmatoxylin give confirmatory results. I therefore think 

 that the drop method of fixation^ when employed in the 

 manner described, gives reliable cytological results in the case 

 of Bacteria.^ On account of the ease with which osmic acid 

 or formol may be employed in this mannei', I have used them 

 more frequently than any other fixatives. 



Another point which calls for comment concerns the use of 

 alcohol after fixation. When " osmic acid " (more correctly, 

 osmium tetroxide, OsO^) is used — either in solution or in the 

 fomn of vapoui* — it is, of course, unnecessary to treat the pre- 

 parations subsequently with alcohol — so far as complete fixa- 

 tion is concerned. In practice, however, I And that films 

 hxed by osuiic vapour or by the drop method adhere to the 

 slide or coverslip better if they are hardened in absolute 

 alcohol for a short time after fixation. When formol is 

 employed, however, it is absolutely necessary to employ 

 alcohol subsequently. As is well known, formaldehyde fixes 

 protoplasmic structures without precipitating them iu an in- 

 soluble form. It is therefore necessary to place the fixed struc- 

 tures in strong alcohol before proceeding further — otherwise 

 fixation may be completely undone in subsequent treatment.- 



' I luay add that Ijeautiful preparations of small flagellates and 

 other Protista may also Ije obtained in this way. 



- Cf. Gustav Mann (1902). This point seems worthy of attention. 

 I note that Swellengreljel (1906) fixes Bacteria by the drop method, 

 \ising formalin. But he does not appear to iise alcohol subsequently, 

 so that many of the appearances which he describes may be due to 

 imperfect fixation. If it is desired to use formalin alone — without 

 using alcohol at all — and to use stains in watery solution, the fixation 

 may be pi-eserved Ijy adding a small percentage of formalin to all the 

 stains, etc., employed after the original fixation. If it is desii-ed to 

 dilute the formalin used in fixation, this should be done with isotonic 

 salt solution — not with water. 



