6 



ROBERT W. HEGNER 



nucleus and half of the cytoplasm. The second offspring was 

 cut into two slightly unequal parts, each containing one nucleus 

 (fig. 5)'. The smaller part was labeled 150.2a and the larger 

 part 150.2b. Both of these parts survived and reproduced. 

 Their immediate offspring were slightly irregular in shape (fig. 

 6) but exhibited spines that could easily be counted. The 

 effects of injuries to the parental shell upon the shape of the off- 

 spring are clearly indicated in figures 7 and 8. The immediate 



?-34 



15-31= 



Fig. 2 Arcella dentate. Pedigree of family 150 showing the number of spines 

 and diameter of the shell of the progenitor of the family and of a few of the 

 ■progeny. Each vertical series of numbers represents a generation. The number 

 preceding the dash is the number of spines and the number succeeding the dash 

 is the diameter of the shell in units of 4.3m each. 



Fig. 4 Arcella dentata. Nuclei, cj^oplasm, cytoplasmic attachments to the 

 inside of the shell, and the chromidial net are shown in this figure as they appear 

 in a stained specimen. X 310. 



progeny (B) of the half specimen (A) is irregular in shape, but 

 the normal shape (C) is usually regained in the next generation. 

 The condition of the shell of the parent evidently has only a very 

 slight influence upon the shape of the shell of the offspring. 



Besides being slightly irregular in shape, these offspring were 

 much smaller than the original, entire parent, and possessed only 

 one nucleus each. Furthermore, the offspring of the smaller 

 part (150.2a) were smaller than those of the larger (150.2b) as is 

 shown in the pedigree in figure 9. This result is similar to that 



