624 SCIENCE PROGRESS 



Fig. 5. — Centropyxis aculeata. Typical specimens from three different 

 strains. (From Root, 1918.) 



Fig. 6. — Centropyxis aculeata. Typical representatives of the high and low 

 branches obtained by selection within a single strain. (From Root, 

 1918.) 



Fig. 7. — A. Arcella dentata. Surface view of a typical specimen, with 

 eleven spines and a diameter of 116 microns. (From Hegner, 1919.) 

 B. Arcella polypora. Surface view of a typical specimen, with six 

 nuclei and a diameter of 119 microns. 



Fig. 8. — Arcella dentata. Diagram showing the most important heritably 

 diverse lines derived from a single specimen by fission. The character 

 used was spine number. The letters indicate the designations of the 

 lines, and the numbers are the mean spine numbers. (From Hegner, 

 *9i8.) 



Fig. 9. — Arcella dentata. a. Binucleate specimen, with 13 spines and 

 diameter of 150 microns, with line showing where it was cut in two. 

 b. Uninucleate specimen, with 9 spines and a diameter of 112 microns, 

 produced by one-half of the binucleate a. c. Binucleate specimen, with 

 13 spines and a diameter of 152 Microns, descended from the uninucleate b. 



