SEDIMEXTING AND CEXTRTFUGTXG. 21 



when not in use. In the course of a large number of experiments 

 nothing has yet indicated that parasitic material from one examina- 

 tion has remained to subsequently contaminate other fecal speci- 

 mens. Parasites that might be suspected of remaining after the 

 cloth had been washed in boiling water might be destroyed by pro- 

 longed boiling or subjection to dry heat experiment shows that 

 eggs so treated are distorted or characterized by the formation of 

 air spaces or oily areas or fresh pieces of cloth could be used each 

 time. This last, however, would be somewhat expensive, as this 

 cloth retails in Washington in half-yard widths at about $5 a yard. 

 It would be cheaper to use the Xo. 120 mesh brass screen. This costs 

 $1.85 a square foot, but would be permanent. 



SEDIMEXTIXG AND ( EXTRI Fl (UNO. 



The feces which pass through the silk sieve are sedimented with 

 plenty of water in the jar. After decanting, the sediment is trans- 

 ferred to a beaker (rig. 1. d) and may now be washed if desired. 

 The entire sediment, or as much as seems desirable, is then centrifuged 

 (see fig. 1, e), repeated centrifuging with the addition of fresh ma- 

 terial adding to the total centrifuge sediment, and may be washed 

 at this point also, as advised by Pepper (1908) and Bass (1909). 

 The writer sometimes washes the material at both points, the second 

 supplementing and completing the first. It is usually sufficient to 

 wash the sediment in the centrifuge. Bass has called attention to the 

 important fact that a centrifuge should only be run the minimum 

 time necessary to bring down the eggs. This time will vary with 

 different centrifuges. With a centrifuge running 3.500 revolutions 

 per minute Bass allows 4 to 10 seconds. I find this enough time 

 with a centrifuge running 1.230 revolutions per minutes. After the 

 material in the two centrifuge tubes is washed in water, one tube is 

 left alone; the water is poured off the other and calcium chlorid solu- 

 tion, with a specific gravity of 1.250. is added to the sediment. After 

 centrifuging, a slide preparation is sometimes made from this tube 

 direct. In most cases the top cubic centimeter is pipetted off. shaken 

 up with 14 cubic centimeters of water, and centrifuged. This is the 

 more satisfactory and certain method. 



PREPARATION' OF SLIDES. 



By means of a long pipette, a drop of sediment is drawn up from 

 the bottom of the tube in which water alone is used, placed on a slide 

 under a cover glass, and examined with a microscope (fig. 1. /). A 

 second slide is made from the other tube. This second slide is either 

 made directly from a drop taken from the surface of the calcium 

 chlorid solution, or from the bottom in case the top cubic centimeter 



