.goS.] JENNINGS— HEREDITY IX PROTOZOA. 437 



set showed a rapid growth in length and breadth. The length now 

 reaches 186.736 microns, the breadth 60.168; both dimensions are 

 considerably greater than the mean of the random sample. Thus, 

 the animals at this age had reached about the average size of the 

 infusoria in a collection of the same descent taken at random. Table 

 VI, (page 412) shows a sample of this same culture taken twenty- 

 four hours earlier, at a time when little division was occurring; the 

 mean length is very nearly the same as that of the young of the 

 present set. The correlation between length and breadth has con- 

 siderably increased. 



Certain peculiar facts are brought out by considering these two 

 sets together (Table X., row 18). Here we have a collection of 188 

 young individuals taken at practically the same time from a small 

 watch-glass culture. The variability and correlation depend in a 

 high degree on the length of time we keep these. If they are all 

 kept three to four hours (row 16) or 4.20 to 5 hours (row 17), the 

 variability in length is about 5 to 6, in breadth about 9. But when 

 we keep part of them for the shorter period, part for the longer, the 

 variability rises to about 12.5 for length and 12 for breadth. Again, 

 the correlation between length and breadth is but .3201 and .5557 

 in the two lots taken separately, but when we take them together the 

 correlation is much greater, rising to .7132. These relations show 

 the important part which may be played by growth in determining 

 observed variability and correlation ; their significance will be taken 

 up again in our general sections on these topics. 



FonrtJi Stage: 12 to 18 Hours Old {Table X., Lot 10). 



From the same culture of the progeny of D from which came the 

 lots last described, but three days later were taken two lots of young, 

 of y^ and 105 specimens, respectively, which were kept, the former 

 to the age of 12 hours, the latter to the age of 18 hours. 



{k) Age 12 Hours {Table XXXIX., and rozcs 20 and 21, Table 

 X.). — There is a still further increase in both length and breadth, as 

 compared with the specimens 4.20 to 5 hours old (see Table X., rows 

 20 and 21). Among the 73 specimens of this lot were two of about 

 the same size wdiich were much smaller than the others (see Table 

 XXXIX.). There is little doubt, I believe, that these are the prod- 



