1922] McDonald: On Balantidium coli and Balantidium suis 247 



In order to avoid the deleterious effects caused by cooling and by 

 increased bacterial action, most of the studies on living organisms were 

 made at the abattoir. When continuous observation over a long period 

 was desired, however, the material was conveyed to the laboratory in 

 thermos bottles and kept in the incubator at 37.5 C. In tfris~manner 

 material could be kept for three days. Ultimate degeneration of the 

 organisms seemed to be due more to the increase in the bacterial con- 

 tent of the medium than to any other cause. During observation either 

 an electric warm stage or the microscope warm oven designed by Long 

 (1912) was employed. Of the several vital stains used, neutral 

 red proved most satisfactory in the differentiation of the neuromotor 

 apparatus. 



Fixation and staining. The following fixatives were used : Schau- 

 dinn's fluid, Zenker's fluid, formalin, osmic acid, and picromercuric 

 fluid (according to the formula by A. D. Drew, used by Yocom, 1912). 

 Quick action was one of the most important factors in the fixation, 

 and was usually obtained by having the killing fluid hot (60-80 C.) 

 and using an amount at least equal to the amount of the material to 

 be fixed. Frequently the action was so nearly instantaneous that the 

 cilia on the killed animals retained their exact relative position (see 

 fig. N). After fixation the material was thoroughly washed, iodine 

 alcohol being used if mercurial salts were present. Material was pre- 

 served in 70 per cent alcohol. 



Before staining, the preserved material was usually concen- 

 trated by elimination of lighter debris by centrifuging and the heavier 

 by sedimentation. Water was found to be a more satisfactory medium 

 for these operations than either alcohol or salt solutions. In case sec- 

 tions were to be made, additional care was taken in the concentration 

 process and then the material was handled according to the methods 

 employed by Metcalf (1909) and by Sharp (1914). 



Iron haematoxylin gave uniformly the best results in staining. For 

 cysts, however, on account of their imperviousness, it was necessary to 

 use Delafield's haematoxylin to which had been added a small amount 

 of acetic acid. In addition to the first mentioned stain, Mallory's con- 

 nective tissue stain was used on sections. 



