Raymond Pearl and Wilbur N. Fuller 
221 
where L denotes the length of the body in centimetres, and S the total number of 
somites. The probable error made in estimating length from number of somites is 
2'004 cms., and in estimating somite number from length the probable error is 
7-718 somites. 
From (i) it is seen that an increase of one somite is associated, on the average, 
with an increase in length of the worm of '6 mm. But, as has been shown above, 
the average length of an individual somite is more than twice this amount 
(1"34 mm.). Now if the individual somites did not increase in size after they are 
added the value given in equation (i) and the average value obtained by dividing 
the total length by the number of somites should agree. The values actually 
obtained lead us to conclude that the individual somite after being laid down 
about doubles in length. This result is of interest as showing the quantitative 
relation of the two factors considered in the growth of this simple segmented 
animal. 
The regressions are sensibly linear. 
3. Variation and Correlation in the Clitellmn. 
In the discussion of this subject the term " position of the clitellum " will be 
used to indicate the number of somites from the anterior end of the body back to 
the first somite which bears any clitellar tissue. It will be recalled that "number 
of somites in the clitellum " is taken to include all somites on which any clitellar 
tissue is present. 
The frequency distributions for these two characters are given in Tables IV. 
and V. respectively. 
TABLE IV. 
Position of Clitellum. 
TABLE V. 
Somites in Clitellum. 
Number of 
Somites 
Frequency 
29 
1 
80 
117 
31 
376 
32 
1 
Total 
495 
Number of 
Somites 
Frequency 
6 
394 
7 
84 
8 
17 
Total 
495 
The very small range of variation in both cases is noteworthy. In the case of 
the position of the clitellum practically all the individuals are included within a 
range of two classes. The number of somites in the clitellum with a total range 
of variation of only three somites has approximately four-fifths of the individuals 
in a single class. 
