ARTICLES 



611 



Jennings (191 6) used the fresh- water Rhizopod Difflugia 

 corona for his investigations of heredity, variation, and the 

 results of selection in uniparental reproduction. The same 

 problems as stated above were involved and similar methods 

 were used in the experiments. Difflugia corona possesses a 

 shell consisting of particles of sand fastened together by a 

 secretion produced by the organism. As shown in Fig. 3, 

 it has six characteristics suitable for selection work : ( 1 ) 

 diameter of the shell ; (2) depth of shell ; (3) diameter of 

 mouth ; (4) number of teeth ; (5) number of spines ; and 

 (6) length of spines. Specimens were brought into the labora- 

 tory and reared on hollow-ground slides under uniform con- 

 ditions. They exhibited the following variations : (1) diameter 



Fig. 3. 



of shell, 107-280 microns; (2) depth of shell, 107-201 microns; 

 (3) diameter of mouth, 56-98 microns ; (4) number of teeth, 

 0-21 ; (5) number of spines, 0-14 ; (6) length of spines, 0-149 

 microns. The presence of diverse strains in nature was deter- 

 mined by the rearing of large numbers of specimens from single 

 parents. For example, in one line containing 117 specimens 

 the spine number ranged from o to 6, whereas in another line 

 of 261 specimens the spine number ranged from 4 to 12. 

 Ranges in variation of other characters were as follows : 

 number of teeth, one line of 55 specimens, from 12 to 17; another 

 line of 52 specimens, from 9 to n ; diameter of shell, one line 

 of 40 specimens, from 121 to 145 microns; another line of 



