16 ROBERT W. HEGNER 



enough to initiate the process of empty shell formation and 

 nuclear doubling, which occurred immediately. 



The other case whose pedigree is shown in figure 15 is some- 

 what similar, A binucleate specimen gave rise to a smaller 

 uninucleate offspring and itself became uninucleate during the 

 process. It then produced another small uninucleate; and 

 followed this by empty shell formation and nuclear doubling, 

 after which its offspring were all normal binucleates. It will 



10-26 (l)C^ 



^15-3lf2) 12-32 f2) 14-35(1) 11-28 (DCI^ 



^^15-36(2) 



14 



,8-24 2 



10-27(1) 



13-31(2) 

 14-33(2) 



15 



Fig. 14 Arcella dentata. Part of pedigree of line 150.2b, showing spine 

 number and diameter of specimens during reversion from the binucleate to the 

 iminucleate condition and back again. E = empty shell. The numbers enclosed 

 by parentheses indicate the number of nuclei present. 



Fig. 15 Arcella dentata. Part of pedigree of line 150.2b, showing spine num- 

 ber and diameter of specimens during reversion from the binucleate to the uni- 

 nucleate condition and back again. E = empty shell. The numbers enclosed 

 by parentheses indicate the number of nuclei present. 



be noted that the first uninucleate was small, differing in this 

 respect from that described above. This may be explained by 

 the failure of the binucleate parent to produce enough cytoplasm 

 to form two binucleates. Outlines of the specimens described 

 in this case of reversion are shown in figure 16. Nuclear divi- 

 sion may in these organisms be initiated when a certain cyto- 

 plasmic mass has been reached, and the lack of cytoplasm may 

 account for the failure of the nuclei to divide in this case. 



