LABORATORY STUDIES AND DEMONSTRATIONS. 217 



vacuole timed (vary by varying temperature), and the effect of 

 tapping the cover-glass noted. It is practically useless to look 

 for fission, for encysted forms, or for the external opening of the 

 contractile vacuole; and the ingulfing of food or passing out 

 of waste matters is rarely seen. The formation of pseudopodia 

 should be carefully studied. After examining the living animals 

 they should be killed and stained with dilute iodine. 



Arcella is almost always, and Difflugia sometimes, found 

 with Amoeba. These forms may be examined for comparison. 



It is desirable also to compare white blood-corpuscles, which 

 may be obtained either by pricking the finger or, better, from a 

 frog or newt. A drop of blood, received upon a slightly warmed 

 elide, should be covered and sealed with oil around the edge of 

 the cover-glass. The white corpuscles are at first rounded, but 

 soon begin to show change of form. (No contractile vacuole, no 

 differentiation into ectoplasm and entoplasin, often no nucleus 

 visible.) 



CHAPTER XII. (INFUSORIA.) 



JParamcecia are almost certain to appear in the earlier stages 

 of the Amoeba cultures, and in similar decomposing liquids or 

 infusions, and to ensure having them a large number of vessels 

 and jars containing an excess of vegetable matter should be pre- 

 pared a month or more beforehand. Their successful study is 

 very easy if they are procured in very large numbers (the water 

 should be milky with them), otherwise it is practically impossible. 

 Three slides of them should be prepared and set aside for a short 

 time (under cover, preferably, in a moist chamber) to allow the 

 animals to become quiet. One slide should contain simply a 

 drop of the infusorial water ; a second the same, with the addi- 

 tion of a little powdered carmine ; to the third add a drop or two 

 of an aqueous solution of chloral hydrate (made by dropping a 

 crystal or two into a watch-glass of water). The first slide 

 should be studied first ; and it will usually be found that after a 

 time the animals crowd about the edges of the cover, often lying 

 nearly or quite still. If this is not the case, the specimens para- 

 lyzed by chloral may be studied. The carmine specimens will 

 show beautiful food-vacuoles filled with carmine ; and by careful 

 study the formation of the vacuoles may be observed. 



