Races of Paramecium. 505 



2 per cent variation. In the search for these variations every in- 

 dividual which appeared unusually large or small, or in which there 

 appeared to be any indication of more — or less — than the usual 

 number of micro-nuclei, was very closely examined. The chances 

 are that an exhaustive study of every specimen upon every slide 

 would decrease rather than increase this ratio. 



The occurrence of even a few variations in the micro-nuclei 

 within pure races is of course in agreement with the facts set forth 

 by Calkins, and shows that there is some basis for his contention 

 that one form transforms into'the other. This matter, however, 

 takes on a very different aspect as soon as the structure of the 

 micro-nuclei is taken into consideration. Maupas and Hertwig 

 both described a characteristic structure of the micro-nuclei of 

 the aurelia form that was different from that of the caudatumform. 

 The drawings which represent the different races already de- 

 scribed (figs. 1-10) show that this difference of structure is very 

 marked. Comparison of fig. 8 (caudatum form) and figs. 9 or 10 

 (aurelia form) brings out the difference in the resting micro-nuclei 

 of the two forms most clearly. The central dark mass, surrounded 

 by a clear "cortical" area, is most characteristic for the aurelia 

 races; actual comparative examination of preparations from the 

 different races impresses one at once with the sharpness of the dis- 

 tinction. Now it was found that in every instance where an 

 individual of a large race had two micro-nuclei these were of ap- 

 proximately the same size and of exactly the same structure as 

 the normal single micro-nucleus characteristic of the race. There 

 was not a single case where either micro-nucleus of a large race 

 ressmbled the micro-nuclei characteristic of the " aurelia" forms. 

 Fig. 8 shows the normal single micro-nucleus from a race of large 

 individuals ("caudatum" forms), and in fig. 7 are shown the two 

 micro-nuclei as they occur in each of the aberrant forms of the 

 same race. And the converse is also true. Fig. 10 represents the 

 structure and arrangement of the normal two micro-nuclei charac- 

 teristic of the ''aurelia" forms. When a single micro-nucleus was 

 occasionally found in an individual of these smaller races (fig. 9) it 

 was in every respect like the normal nuclei of the ''aurelia" forms 

 and never resembled in any way the micro-nucleus of the " cauda- 



