ANIMAL PARASITES 49 



Centrifuging. — An improvement on the method of simple 

 sedimentation is thoroughly to emulsify in water a sample the 

 size of a walnut, strain out the coarse particles, and then centrifuge 

 for the purpose of giving a rapid and certain concentration. 



Treatment with Dense Liquids. — Advantage may be taken of 

 the fact that parasite eggs are of specific gravity different from 

 that of the other constituents of the material to be examined. 

 In hookworm work a small quantity of feces is placed in a vial 

 three-fourths full of saturated solution of salt (NaCl), thoroughly 

 shaken, and allowed to stand for an hour; then a drop from the 

 surface of the fluid is examined. If the concentration is correct 

 all of the eggs rise to the surface and large numbers may 

 be found in a single drop. The process is much facilitated 

 by forcing the coarse float below the surface with a disk of No. 

 steel wool. 



The so-called Willis method is now widely used in hookworm 

 campaigns. A saturated solution of NaCl is prepared in boiling 

 water and allowed to cool. One or two grams of the fecal sample 

 are then thoroughly mixed with this concentrated salt solution 

 and the container then filled to the brim with it. A clean 

 glass slide is placed in contact with the film and allowed to 

 stand for 15 minutes. It is then carefully removed, inverted, 

 and examined under the low power of the microscope for 

 adhering eggs. 



The various methods of treatment with dense liquids will not 

 reveal the presence of operculate eggs such as those of most 

 flukes and of the fish tapeworm of man. 



PRACTICAL WORK 



Prepare an emulsified sample as above described or mount 

 under a square cover glass a small drop of sedimented fecal 

 sample preserved in 10 per cent formalin solution. 



Beginning at one corner of the preparation pass systematically 

 from end to end under the microscope until the whole field has 

 been covered. Distinguish carefully between fragments of 

 plant tissue, partially digested muscle and other food, plant 

 spores (see Fig. 18), and the eggs, larvae and cysts of parasites. 

 Vary the lighting of your preparation by the use of the iris 

 diaphragm and note the great role which this procedure plays in 

 the detection and identification of significant objects. 



