284 
ZOOLOGY: R. W. HEGNER 
that differed constantly and markedly in their fission rate. Jennnings has 
shown that the descendants of a single specimen of Difflugia may be separated 
into a number of diverse lines that differ from one another in their heritable 
characteristics. The work herein described is part of an investigation that is 
being made of the species problem in the genus Arcella and the principal prob- 
lem attacked is : Can heritably diverse lines with respect to spine number and 
diameter of shell be distinguished among the descendants of a single specimen 
of Arcella dentata produced by simple fission? 
Arcella dentata (fig. 1) is a microscopic protozoon belonging to the lowest 
class, the Rhizopoda. It is as simple as any animal organism it is possible to 
FIG. 1. OUTLINE DRAWINGS OF SPECIMENS OF ARCELLA DENTATA BELONGING TO FAMILY 
A, The progenitor of the entire family; B, a typical member of the low line E; C, a typical 
member of the high line A ; D, the small progenitor of the line EM; E, a small specimen from 
the line ED; F, the largest specimen from the line ED. 
obtain that has measurable characteristics. It varies in diameter from 73 
microns to 150 microns and in spine number from 7 to 20. It multiplies vege- 
tatively and rapidly and the characteristics of the shell are not modified by 
growth or by the environment, and are heritable but variable. In all, 6474 
specimens were studied. Of these 171 were collected from a pond on the 
campus of the Johns Hopkins University at Homewood, Baltimore; 746 were 
reared from 70 of these specimens; and 5557 were obtained from the single 
specimen numbered 58. The number of generations represented by the prog- 
eny in family 58 was 69 and the average interval between generations was 
two and one-half days. 
F 
NO. 58. X 207 
