Races of Paramecium. 501 



In the determination of the number of micro-nuclei about ten 

 to twenty individuals upon each slide were examined, section by 

 section. The ones picked out for this careful study were mainly 

 those in which the plane of the section was nearly parallel with the 

 long axis of the body; specimens which appeared to be most normal 

 in shape and in the appearance of the cytoplasm, etc., were the only 

 ones included. That is, if specimens were in an abnormal state 

 as regards cytoplasm or meganucleus, they were not considered. 

 The animals were killed and sectioned at different periods, some- 

 times several weeks or a month apart, and again within a few days 

 of each other. The small number carefully examined upon each 

 slide and the large number of slides thus treated would seem to 

 give a very fair average of the conditions of the entire culture for 

 the entire period of the investigation (four months). While no 

 exact tally of the individuals carefully examined was kept, it is cer- 

 tain that one hundred or more for each culture is below rather than 

 above the probability. Besides, several times as many from each 

 culture were gone over less carefully with confirmatory results. 



In brief the results obtained are these : in the races labeled 

 D and L2 only a single micro-nucleus was present. In the strains 

 c, i, ko, khaiGi, Co there were two micro-nuclei in each. The struc- 

 ture and staining reaction of the single micro-nucleus was like 

 that commonly described and figured. In the races with two 

 micro-nuclei these agreed in almost every detail with the descrip- 

 tions given by Maupas and Hertwig and summarized earlier in 

 this paper. Figs. 1 to 11 with their accompanying explanations 

 will serve to show the characteristic appearance and position 

 of these two sorts of micro-nuclei. In addition to the size and 

 structure of the micro-nuclei the external characters of the races 

 agreed in general with the descriptions given by earlier writers for 

 the "aurelia" and ''candatum" forms. 



If now the races be arranged in the order of their size as found 

 by Jennings ('08), we have (the largest first); L2, D2, k, Ci, c, i, 

 and of these Lo and D had each a single micro-nucleus, while the 

 other races each had two micro-nuclei (I now found that ko, 

 and khaia-i were of the same race), i.e., the larger forms had a single 

 micro-nucleus while the smaller ones had two micro-nuclei. A com- 



JouRNAL OF Morphology, Vol. 21, No. 4. 



