504 H. S. Jennings and George T. Hargitt. 



parispn with the earher suggestions of Jennings is of interest in this 

 connection. He says (p. 500): ''I beheve it will be found that 

 most Paramecia can be placed in one of the two groups that we 

 have called 'caudatum' and 'aurelia' . . . most lines will 

 have a mean length either below 145 microns or above 170 mi- 

 crons," '^ Furthermore, I am inclined to beheve that those belong*- 

 ing to the smaller group (mean length below 145 microns) will 

 be found to have as a rule two micro-nuclei; those belonging to the 

 large group but one micro-nucleus." The prophecy is thus most 

 strikingly confirmed. It is important to remember that these 

 results were not obtained from a single examination of a few indi- 

 viduals at one time. They represent the results of the examina- 

 tion of from a hundred to several hundred random individuals 

 taken from the cultures at intervals of a few days in some cases, 

 and in other instances at intervals of a month or two. Further, 

 since no attempt was made to have the cultural conditions the 

 same, it can not be claimed that the results are dependent upon 

 the constancy of their environment. While hay infusion was used 

 for the culture medium in each case, the hay came from different 

 sources, was sometimes boiled and sometimes fresh. The amount 

 of water and of hay used, and consequently the strength of the 

 infusion, varied considerably. This, it is believed, makes a nearer 

 approach to the natural conditions than would result from the 

 maintenance of constant and uniform conditions in the culture 

 media. But in spite of the conditions, which were different at 

 different times, the number of micro-nuclei was constant. The 

 number of micro-nuclei had been determined three years earlier by 

 Jennings ('08, p. 500) for two of these races, D and c, and found to 

 be the same as at present. 



Constant though these races on the whole were in the number of 

 micro-nuclei, there were a few individuals which varied. Thus, 

 the races which have the single micro-nucleus show occasional indi- 

 viduals with two micro-nuclei; the smaller forms less often have a 

 single micro-nucleus instead of the normal number, two. The 

 number of variations is very small, however. Thus there were 

 found in all the races only eighteen variants, which out of about a 

 thousand individuals carefully examined gives a ratio of only about 



