84 ROBERT W. HEGNER 



nuclei occurred. Thus, as shown in table 16 and on pages 44 

 and 45, offspring whose nuclear number is less than that of the 

 parent are almost invariably smaller than the parent, and 

 when nuclear doubling occurs, as, for example, when a parent 

 with 3 nuclei gives rise to progeny with 6 nuclei, the imme- 

 diate result is a more decided increase in the diameter of the 

 latter. 



Bisected specimens of A. polypora belonging to family ap. 5 

 behaved very much as did those of A. dentata except that the 

 acquisition of new nuclei was not accompanied by empty shell 

 formation. The results of the cutting operations confirm those 

 obtained from the study of uninjured animals, since specimens 

 with 1 or 2 nuclei were smaller than those with 2 or 3 and an 

 increase in nuclear number was always accompanied by an 

 increase in size. In A. polypora, as in A. dentata, there was 

 a tendency to return, after the operations, to the condition 

 'normal' for the line, and it was possible to procure only a few 

 specimens with 1 or 2 nuclei, since such organisms almost imme- 

 diately acquired a number of nuclei near the average for the 

 line from which they had been derived. Other families of A. 

 polypora were similar to families ap. 5 and ap. 34 in their nucleo- 

 cytoplasmic relations as described on pages 51 and 52. 



Arcella discoides. In this species the variations in size were 

 small as compared with those encountered in A. polypora and 

 more nearly like those described for A. dentata. This is easily 

 explained by the fact that the variation in nuclear number was 

 never more than one. Uninucleates resulting from the bisec- 

 tion of a normal binucleate specimen reacted, as shown in figure 

 42, very much as did those of A. dentata, except for the absence 

 of empty shells at the time of nuclear doubling. 



Arcella vulgaris. In this species further evidence was 

 obtained of a definite nucleocytoplasmic mass relation, since, 

 as indicated in table 28, the binucleate specimens in family 

 av. 7 had a mean diameter of 28.86 units of 4.3 ju each, whereas 

 the trinucleate specimens had a mean diameter of 30 units. 



