NUCLEAR VOLUME AND LIFE-CYCLE OF HYDATINA 



309 



NUCLEAR VOLUME AND AGE OF INDIVIDUAL 



From the rotifers fixed in early life, at middle age, and just 

 before the end of the reproductive period, as described in the 

 introduction, were obtained the measurements described in this 

 section. The cells from the three tissues are described separately. 



Yolk gland. Three lots of rotifers were fixed for the study of 

 cells in relation to age, each lot being divided into three groups 

 of different ages. The relative nuclear volumes of all cells in 

 each group have been averaged, and the means given in table 6. 

 In every experiment the nuclear volume is greater the older the 

 animal. The combined results shown in the last Une of the table 

 show differences that are many times their probable error, and 



TABLE 6 



Relative nuclear volume in the yolk gland of Hydatina senta in young, middle-aged, 

 and old specimens. The data are derived from three similar experiments 



there seems no question that they are significant. The relative 

 nuclear volume in the yolk gland increases with the age of the 

 individual. 



The manner in which this increase of relative nuclear volume 

 is brought about, whether by increase of the absolute nuclear 

 volume or decrease of the volume of the cytoplasm, or both, is 

 of importance. All drawings of the yolk gland were made on 

 the same scale, so that absolute sizes can be determined (table 7) . 

 The mean nuclear sizes, expressed in square inches on the draw- 

 ings, for the three age-groups are very much larger at middle age 

 than in early adult life, but decrease later without, however, being 

 reduced to the size of nuclei in young adults. The smallest dif- 

 ference shown in this comparison, that between middle-aged and 



