304 A. FRANKLIN SHULL 



life-cycle exert their influence almost immediately (Shull, '12), 

 and one would suppose that an internal agency would operate 

 with similar promptness. 



Stomach-intestine. The average relative nuclear volume of the 

 cells of the stomach-intestine for the nineteen generations in which 

 periods of male-production were well marked is shown in table 3. 

 The data are graphically shown in figure 1, third curve, together 

 with the curve of male-production (first curve) for the purpose 

 of comparison. The curve of nuclear volume shows a small but 

 evident decline in the latter part of the period studied. If the 

 nineteen generations be divided into three approximately equal 

 parts, the average relative nuclear volume in the first two of these 

 is practically the same; but in the third division (the last six 

 generations) the relative size of the nucleus appears to be signifi- 

 cantly less. To determine whether this relative decrease is due 

 to an absolute decrease in the size of the nucleus or to an absolute 

 increase of the volume of cytoplasm, or to both, the absolute 

 sizes of all intestinal nuclei of the nineteen generations have 

 been collected. To reduce the probable errors, these generations 

 have been divided into three periods, the first six generations, 

 the seventh to thirteenth, and the last six generations, respec- 

 tively. The mean absolute nuclear sizes (square inches in the 

 drawings) of the nuclei in these three periods were found to be 

 as follows : 



Size of intestinal nuclei, first six generations 0.428 ± 0.0047 



Size in seventh to thirteenth generations 0.432 ± 0.0083 



Size in last six generations 0.400 ± 0.0059 



The first two of these periods of time were marked by intestinal 

 nuclei of practically the same size, but the nuclei in the third 

 period appear to be significantly smaller. This decrease of abso- 

 lute nuclear size is approximately what is required to explain the 

 decrease of relative nuclear size shown in table 3 in the same six 

 generations, indicating that the mean volume of the cells as a 

 whole remained nearly constant. 



The variation in nuclear volumes in the stomach-intestine 

 shows no relation to the 'waves' of male-producers shown in the 



