PKOTOZOA 7 



ring is cemented. A small flat drop of the material to be examined 

 : is placed upon a cover-glass, which is then arranged upon the glass 

 slide with the drop immediately over the hollow. To render this little 

 chamber air-tight, a ring of vaseline is previously painted round the 

 hollow in the glass slide, and into this the cover-glass is gently pressed. 

 Attempts to obtain pure cultures of Protozoa have frequently been 

 made, but in the nature of things such experiments could only be 

 successful, in the case of species which are nourished entirely by 

 endosmosis. But even among these, certain very delicate cell para- 

 sites, such, for instance, as the Coccidia, are not, as far as our present 

 experience goes, susceptible of cultivation in artificial media. Up to 

 the present, pure cultures have been unsuccessful, except in the case 

 of the flagellate blood parasites (see later, under special heading). On 

 the other hand, certain of the Protozoa which depend for their nourish- 

 ment upon solid matter, such as the Amoebae, may be fed with bacteria, 

 and in this way cultivated in a form the practical value of which 

 approximates very closely to that of the true pure culture (see later, 

 under special heading). 



The study of the more minute cytological details, and especially 

 the structure of the nucleus, necessitates the colouring and fixing of 

 the organisms. This may be done in the form either of sections or of 

 cover-glass preparations. It should not be forgotten that, under 

 certain conditions, fixing and colouring may have a diagnostic value. 

 With regard to procedure, it is beyond the limits of this work to give 

 more than an account of the methods which are of particular value in 

 examining Protozoa. For all other information the student is referred 

 to the text-books on the subject. 



The most important fixing mixtures are the following : 

 Alcoholic Solution of Mercuric Chloride (Schaudinn). A mixture 

 of 1 part absolute alcohol and 2 parts saturated solution of mer- 

 curic chloride. The latter ingredient is obtained by dissolving per- 

 chloride of mercury in boiling normal saline solution (O75 gramme 

 NaCl in 100 c.cm. distilled water) in such proportions that when the 

 liquor cools a few crystals of perchloride of mercury are deposited. 

 The proportion of mercuric chloride to alcohol need not be exact. 

 I usually mix them by the eye in a test-tube. The mixture should be 

 used warm (about 50 to 60 C.) and should take effect upon cover- 

 glass preparations in about two to five minutes. Larger objects take 

 proportionately longer. The specimens are then washed, first in 

 50 per cent., then 70 per cent, alcohol. They are next transferred 

 to iodine alcohol, that is, 70 per cent, alcohol to which sufficient 

 tincture of iodine or, better still, Lugol's solution (aqua destillata 100, 



