LABORATORY STUDIES AND DEMONSTRATIONS. 217 



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

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

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

 contractile vacuole; and the ingulhng of food or passing out 

 of waste matters is rarely seen. The formation of pseiid(»i)odia 

 should he carefully studied. After exanunhig the living aTiimals 

 they should be killed and stained with dilute iodine. 



Arcella is almost always, and Diffliujia sometimes, found 

 Avith Arnceha. These forms may be examined for comparison. 



It is desirable also to compare white l)lood-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 

 slide, should be covered and sealed with oil aroimd the edtre of 

 the cover-glass. The white corpuscles are at first rounded, l)ut 

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

 differentiation into ectoplasm and entoplasm, often no nucleus 

 visible.) 



Chapter XII. (Infusoria.) 



Parammcia are almost certain to ap2:>ear in the earlier stages 

 of the ATYioeha cultures, and in similar decomposing licpiids or 

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

 and jars containing an excess of vegetable matter should be i)re- 

 23ared a month or more beforehand. Their successful study is 

 very easy if they are procured in "cery large miirLbers (the water 

 should be milky with them), otherwise it is practically im]")ossible. 

 Three slides of them should be pre2)ared 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 (h*oj) or two 

 of an aqueous solution of chloral hydrate (made by droi)i)ing a 

 crvstal or two into a watch-o-lass of water). The tirst slide 

 should be studied iirst; and it will usually be fiamd 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 sj)ecimens j)ara- 

 lyzed by chloral may be studied. The carmine sjiecimens will 

 show beautiful food-vacuoles tilled witli carmine; and bv careful 

 study the formation of the vacuoles may be observed. 



