432 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



together the variation is great in proportion to the mean length. 

 While the statistical data are themselves open to other interpretations, 

 observation of the changes in living individuals, as described earlier, 

 shows that this explanation is the correct one. 



The absolute variation of the young, as shown by the standard 

 deviation, is less, as might be expected, than that of the culture as a 

 whole, though the difference is not great. 



4. The variation in breadth, both absolute and relative, is less in 

 the young than in the culture as a whole. The fact that it is still 

 considerable perhaps indicates that changes in breadth are taking 

 place during growth. To this we shall return immediately. 



5. The correlation between length and breadth is negative in the 

 young, while in the culture as a whole it is positive. In the former 

 the coefficient is — .3138; in the latter it is -|- .4282. 



The fact that the correlation is negative in young specimens 

 (greater length associated with less breadth) indicates that while the 

 animals are growing in length they are becoming more slender. 

 With an increase of 10 microns in length the decrease in breadth is 

 .757 micron. If we group together the unseparated halves (124 in 

 number) with the separated ones (59), we find that the negative 

 correlation between length and breadth is still greater, becoming 

 — •.3625 (see row 4, Table X.). 



6. The mean ratio of breadth to length (" mean index ") is much 

 greater in the young than in the random sample. In the former the 

 breadth is 44.037 per cent, of the length; in the latter but 25.084 per 

 cent. If we include the unseparated halves with those under five 

 minutes old, the breadth is 54.080 per cent, of the length (row 4, 

 Table X.), while in the unseparated halves alone it is 59.166 per 

 cent., and in the earliest stages of the unseparated halves it is 61.530 

 per cent, (see Table VIIL, rows 4 and 5). There is thus a steady 

 reduction of the ratio of breadth to length ; to this is due the negative 

 correlation of the two, when those of different ages are thrown 

 together. 



(d) Age to ip Minutes ( Tables XXXI. and XXXIL).— From 

 another culture composed of descendants of the individual D, speci- 

 mens were taken on June 14 and kept to several different ages. The 



