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SCIENCE PROGRESS 



differed in the characters mentioned. First a study was made 

 of the number of nuclei and the diameter of the shell. Two 

 representative families will be described here. In family 5 

 the nuclear number ranged from 3 to 7. Specimens with one 

 and with two nuclei were obtained by cutting trinucleate 

 specimens in two. Changes in nuclear number within the 

 members of the family were continually taking place. Thus 

 a specimen with 3 nuclei sometimes produced offspring with 

 4, 5, or 6 nuclei, and a specimen with 5 nuclei sometimes gave 

 rise to descendants with 3, 4, 6, or 7 nuclei. The most striking 

 phenomenon revealed by these organisms was the close cor- 

 relation between nuclear number and size. In every case, 

 when an increase in nuclear number occurred, it was accom- 

 panied by an increase in the size of the specimen, and likewise 

 a decrease in nuclear number always was accompanied by a 

 decrease in size. The members of family 34 were provided 

 with from 5 to 10 nuclei. Changes in nuclear number were 

 accompanied by changes in size as in family 5, but the members 

 of family 34 with a certain number of nuclei were always 

 smaller than those in family 5 with the same number of nuclei. 

 The differences are indicated in the following table : 



These results led to a study of the chromatin masses within 

 the nuclei of members of the two families, and to the conclusion 

 that in this species, as in Arcella dentata, size depends upon the 

 quantity of chromatin present. Measurements of the chromatin 

 masses of specimens in families 5 and 34 showed that in both 

 families a certain size is correlated with a certain amount of 

 chromatin, and that increases and decreases in size are accom- 

 panied by corresponding increases and decreases in chromatin 

 mass. The following table shows that this is true : 



Average 



Diameter of 



Shell in 



Microns. 



112*19 



116-67 



125-56 



Chromatin Mass in cjnm. 



