578 lcTrande loss woodruff 



The length of the individual experiments varied with the 

 development of the organisms in the culture medium flasks. 

 Obviously, it was necessary that there should be very heavy and 

 essentially similar growths of both the paramaecia and the hypo- 

 trichs before the test of the media began, and frequently the 

 medium had to be discarded and a new lot started, owing to the 

 failure of heavy growths to develop. The more rapid rate of 

 division of the hypotrichous forms rendered it expedient to seed 

 the cultures with a few less individuals of these forms than of 

 paramaecia in order to have the development of the two flasks 

 synchronous. Again, the actual experiments could be continued 

 only while heavy growths of the respective species persisted and 

 consequently the length of the experiments varied, though a great 

 majority comprised five days. In many cases it was found impos- 

 sible to conduct series of paramaecia and hypotrichs simulta- 

 neously — ^but all data which are directly compared to determine 

 the results, were run at the same time and consequently tempera- 

 ture changes are without influence. 



The general conduct of the experiments was similar to that 

 employed in my former work on the subject and therefore it is 

 unnecessary to describe it in detail again. In each individual 

 experiment four animals were isolated with the aid of a Zeiss 

 binocular microscope and placed on separate depression slides and 

 bred in the medium to be tested, which was supplied fresh daily 

 during the period of the experiment. The average rate of division 

 of these four lines, again averaged for the number of days of the 

 experiment, afforded the data on which this paper is based. 



The details of a couple of typical experiments will best illus- 

 trate the modus operandi. On January 18,' 1912, sixteen para- 

 maecia were isolated from the main pedigreed culture and each 

 one was placed on a clean depression slide. Four of these, were 

 supplied with medium from flask P from which the paramaecia 

 had been entirely removed,^ four were supplied with culture 

 medium from flask H from which the hypotrichs had been care- 



3 The infusoria were removed by filtering the medium through filter paper and 

 then picking out with a pipet any of the animals which happened to pass through 

 the paper. Medium was also filtered in order to make the treatment the same for 

 all the types of culture media. 



