290 EGBERT T. HANCE 



pressure at the center tends automatically to lessen as the sur- 

 face tension slowly draws the liquid from its close adhesion to 

 the slide and rounds it up. This takes a sufficient length of time, 

 however, to allow accurate observations to be made under the 

 compound microscope. Occasionally the pressure is relieved too 

 quickly and the paramoecium is given enough fluid to move 

 about in, which makes observation with the high power impos- 

 sible and the processs has to be repeated. As soon as the ex- 

 amination has been made, a drop of fresh hay infusion is added. 

 To drop this liquid immediately on top of the animal after it 

 has been under pressure may frequently crush it and the safer 

 way is to allow the fresh fluid to flow from the side into the 

 drop containing the animal. The paramoecium may then be 

 picked up in a pipette and placed in its respective watch glass. 

 When examining paramoecia in pure lines a separate pipette is 

 used for each line. 



Under the 16 mm. lens when the animal is compressed, the 

 vacuoles stand out with diagrammatic clearness appearing as so 

 many holes punched through the cell and as a rule are to be found 

 close to one side and almost invariably lying in a straight line. 

 This latter characteristic was one of the criteria for differentiat- 

 ing between the contractile vacuoles and food vacuoles when 

 these were so numerous as partly to obscure the former. The 

 comparative lack of refraction of the pulsating organs was an- 

 other basis of distinction. When, however, the refraction of the 

 food vacuoles was almost identical with that of the contractile 

 vacuoles under the low power, the 4 mm. lens was ST\mng into 

 place and the contraction of the supposed contractile \'acuoles 

 was watched for. It has been observed that the vacuoles in 

 animals raised in watch glasses are more difficult to see than they 

 are in animals living in larger amounts of medium. In the 

 watch glasses the food vacuoles have a lack of refraction which 

 gives them an appearance very much like the contractile vacuoles 

 whereas the food vacuoles in animals raised in the large culture 

 jars frequently appear very nearly black. 



Cytological technique. For measuring and drawing the whole 

 paramoecium Worcester's fluid was used (10 per c5nt formalin 



