Races of Paramecium. 



513 



10 to 11 mimites, killed in Worcester's fluid, and measured.'* 

 Ten to fifteen specimens of each race were measured at this age. 

 The mean length and breadth for the different races examined 

 are given in table 2. 



TABLE 2 



Mean length and breadth in microns for a number of diverse races at the age of 

 ten minutes. 



On comparing the lengths given in this table with those given 

 in table 1, it is evident that the races show at the age of ten minutes 

 the same order of relative size as in the samples taken from exten- 

 sive cultures including all ages, but with one apparent exception. 

 The race 20 falls below D in length in the present table, whereas 

 in table 1 it stands notably above it. But this peculiarity of race 

 20 is fully explained by the facts observed in taking the measure- 

 ments. Twenty-four hours after the animals were placed in fresh 

 "J standard" infusion there were no dividing specimens in 

 race 20, though there were in all the other races. A repetition 

 of the experiment gave the same result. It was found that the 

 only way to get dividing specimens of race 20 was to examine the 

 culture several hours earlier than in the case of the other races. 

 Thus the dividing specimens of 20 were taken but about eighteen 

 hours after their introduction into the fresh infusion, the other 



* The age ten minutes was selected for comparison with the measurements of 

 Popoff ('09) at that age, in studying the relative volume of nucleus and cytoplasm. 

 It had been my intention to make a similar study of nucleus and cytoplasm for 

 the diverse races; but it was found that at this age the nuclei were still very irreg- 

 ular, so that the plan was not carried out. It will be undertaken later. 



