REPRODUCTION OF THE HYPOTRICHOUS INFUSORIA 217 



ences were evident in the life history of these three cultures in 

 the way of division rate and length of time which they lived, 

 they all showed what might be termed typical 'life cycles' ending 

 with the death of the organisms. 



d. Culture Obtt. A few days after the starting of the Ob culture, 

 October 22, 1911, the animals from one of the stock slides 

 were placed in a test tube containing some of the beef extract 

 medium. This proved to be a favorable environment for the 

 animals and the culture was continued by the following methods: 

 The sterile beef extract medium is kept in test tubes, about 25 

 cc. in each. Into such a tube is introduced a little culture me- 

 dium containing some of the animals. The animals thus intro- 

 duced multiply very rapidly, if the conditions are favorable, and 

 in about four or five days the maximum number is usually pres- 

 ent. After this time the number of animals present gradually 

 decreases, due chiefly, it is supposed, to a lack of sufficient food 

 supply. When about the maximum number of animals is pres- 

 ent, a little of the medium containing some of them is intro- 

 duced into a fresh tube of the beef extract medium and thus 

 the culture is continued. The bacteria which are introduced at 

 the time of inoculation also multiply very rapidly in the fresh 

 medium and furnish the food supply for the animals of the cul- 

 ture. The test tubes containing the animals are examined with 

 great care daily by means of a Zeiss binocular microscope. By 

 the use of such an instrument it is quite easy to examine the 

 individuals contained in a small test tube very thoroughly and 

 to be certain that conjugation is not occurring. 



This mass culture in the test tubes was continued on the 

 beef medium from October 22, 1911, until May 15, 1912. It was 

 noted at this time that the animals in the culture were not mul- 

 tiplying as rapidly as usual and it was thought that a change 

 of medium might be beneficial. Accordingly a large number of 

 media of different kinds and varying strengths were made up 

 from materials gathered from ponds and also from hay and 

 grasses taken from different localities. These infusions were 

 boiled to prevent contamination of the culture by foreign species 



