274 DEAN A. PACK. 



The number of living organisms appearing was so large in the 

 first cultures, that contamination was suspected. Consequently 

 collections for checking were made April 30, 1918, under all the 

 precautions that one takes in the collection of water samples for 

 bacteriological examination. The material was examined and 

 found to contain the two ciliata described later in this article 

 and some other forms in common with the September material. 



Dilution was effected gradually and accurately, by a method 

 of continuous diluting. The process consisted in adding enough 

 distilled water to a culture to lower its density .0025 or one 

 fourth of one per cent, every 24 houi s. This method was adopted 

 for several reasons; first, one does not destroy delicate forms 

 that would perish when plunged into water of only one tenth 

 the original density; second, one has before him a continuous 

 picture of the changing life; and third, one deals with the off- 

 spring of only a few individuals. 



The moist chamber made by the use of the hollow-ground 

 slide was found to be an advantage in the study of these forms. 

 It was made by placing a small quantity of culture, that showed 

 an equal balance between animal and plant life, on a clean covei 

 slip; inveiting this over a deep hollow-ground slide, and sealing 

 with vaseline. If small cultures are desired several may be 

 made on one cover slip by means of the platinum loop. In this 

 case a drop of culture is placed in the hollow-ground slide to 

 prevent evaporation. A mechanical stage greatly facilitates 

 the study. 



Cultures with a predominance of animals or plants will show 

 the effect of different gases upon the form and habits of both 

 animals and plants. Not only can the effect of gases and liquids 

 on living forms be studied to good advantage, but this prepara- 

 tion makes possible accurate study of responses to light, dark- 

 ness, he"at, etc. Some of these preparations were made fifty 

 days ago and are at date in good condition. 



GENERAL DISCUSSION. 



Diluting the culture medium produced certain definite changes 

 in these two ciliata. These protozoans were found to vary with 

 a change in the supply of food, oxygen, carbon dioxide, and light;. 



