NUCLEAR VOLUME AND LIFE-CYCLE OF HYDATINA 301 



The personal equation. While the drawings and measurements 

 were made by several persons, precautions were taken to prevent 

 error from this source. The drawings to be made were listed by 

 the author by means of stage readings and other suitable desig- 

 nations, but in such a way that no one else could know the source 

 of the animal whose cells were to be drawn. The drawings and 

 measurements were therefore made by persons wholly ignorant 

 of the bearing which any given cell would have on the correctness 

 of Hertwig's theory. 



Furthermore, all drawings and measurements which were to be 

 compared with one another in any way whatever were made by 

 the same person. Whatever idiosjmcrasies this person possessed 

 must have affected both sides of the comparison equally and in 

 the same direction. 



ACKNOWLEDGMENTS 



In the labor which this study entailed I have been ably assisted 

 by Margaret B. Shull, Roberta Deam, and Marguerite Swanson. 



NUCLEAR VOLUME AND PERIODICITY OF MALE-PRODUCTION 



The periodicity in the production of males exhibited by the 

 line of rotifers used for this study has been shown in table 1 . The 

 relative nuclear volumes in animals from different parts of this 

 line are here recorded for the three tissues separately. 



Yolk gland. Over a thousand measurements, each involving 

 one to three nuclei, are available for determining the relative 

 nuclear volume of the yolk gland in the nineteen generations 

 through w^hich the line was bred. These have been averaged 

 for each generation separately, with the result shown in table 2. 

 The averages are plotted in the second curve of figure 1, in the 

 first curve of which is given the percentage of male-producers in 

 these nineteen generations. 



There is an irregular but pronounced increase in the relative 

 nuclear volume through the series of generations. The differ- 

 ences between the early generations and the late ones are large 

 as compared with their probable errors. To determine whether 

 this increase is due to absolute increase of nuclear volume or to 



