92 • CLAUDE W. MITCHELL 



which it passes, and the cause or causes of the transition from this 

 type to the better known, more predominating, humped form. 

 The following experiments have been conducted at the Zoological 

 Laboratories of the University of Nebraska, under the supervision 

 of Dr. Powers, to whom I wish to acknowledge my indebtedness 

 and extend my most sincere thanks, for his interest, valuable sug- 

 gestions and aid. 



EXPERIMENTAL CONDITIONS 



In practically all the earlier experiments Paramecia were 

 used as food. In the later experiments, various Protozoa, especi- 

 ally Oxytricha and Euglena, as well as the rotifers Brachionus 

 bakeri, B. urceolaris, and Hydatina senta, were employed. The 

 Paramecia used were taken from a tub in which a general culture 

 of this organism had been maintained for nearly two years. The 

 water in this tub was stale and the odor of decomposition strong. 

 But as it seemed to be a favorable medium for Asplanchna, and 

 as the quantity present sufficed for numerous experiments, it 

 was used as the culture medium for the rotifers. Fresh tap water 

 replaced from time to time the amount thus removed from the 

 general Paramecium culture, but the quantity added was not large 

 enough to afTect appreciably the character of the culture medium. 



Isolation cultures were conducted in all pedigree series. But 

 mass cultures were also employed for certain experiments. As 

 containers watch glasses were used of two sizes. At the close 

 of each experiment these glasses were throughly cleansed and 

 dried. Different pipettes were used for different purposes, those 

 for food never being employed for transferring individuals. All 

 transfers were made under a hand lens of 8 X . This gave suffi- 

 cient magnification to enable the observer to see each individual 

 clearly. As only one individual was transferred at a time it was 

 considered unnecessary to heat either pipettes or dishes before a 

 second use. Tests were nevertheless made at different times to 

 discover whether or not a rotifer could possibly have been carried 

 by accident in the dry dishes; but no such possibility was detected 

 and it is safe to assume that no error in number or pedigree was 

 made by this means during the course of the experiments. 



