64 ROBERT W. HEGNER 



The three cases just described indicate that within these fam- 

 iHes of A. discoides derived from one wild specimen by fission, 

 there is a rather definite 'normal' condition as regards nuclear 

 number, shell shape, and shell size, which may be disturbed tem- 

 porarily, but is very soon regained. 



c. The reproduction of uninucleate pieces 



The data obtained from a cutting experiment make these 

 results even more conclusive. A specimen from family ad. 3 was 

 cut into two approximately equal pieces, each of which con- 

 tained one of the two nuclei. The pedigree of these two pieces 



33 f2); 



i33(l) 



29(3) —30(2) 



-21(1) 



^^33(1) 22(1) 27(2): 



30(2) 31(2) 



-29(2) 



Fig. 42 Arcella discoides. Family ad. 3. Pedigree showing diameter and 

 nuclear numbers of specimens derived from the two halves of a bisected specimen. 



is given in figure 42, in which the large numbers indicate the 

 diameter in units and the small numbers the number of nuclei. 

 The first offspring of one piece was supplied with 2 nuclei, but the 

 parental piece retained only 1 nucleus, and the two succeeding 

 progeny had only 1 nucleus each. Evidently the nucleus of the 

 parental piece divided into two and then one of these divided 

 again before the first offspring was produced. Then when the 

 first fission occurred two of the three nuclei thus formed were 

 contributed to the offspring. The third generation exhibited an 

 increase in size to near the mean diameter of the family, which was 

 30.69 units. In the history of the other piece nuclear doubling 

 took place in the second generation and was accompanied by 

 a sudden increase of 5 units. The mean diameter of the family 

 was reached in the next generation. No empty shells were 



