II. 2 GROWTH 71 



the third to the fifth clays, more slowly from the fifth to the 

 eighth days. After this the rate becomes negative, as the skeleton 

 of the Plnteus is used up by the developing urchin. The 

 curves of change of rate of growth as constructed from 

 Vernon's figures are shown in the chart (Fig. 38). 



In Carcinns moenas a gradual decrease in the growth-rate of 

 the frontal breadth can be ascertained from Weldon's data. 



We have next to consider another feature of growth, the 

 alteration of variability. The facts at our command are derived 

 from a study of Echinoid larvae (Vernon), Duck embryos 

 (Fischel), Guinea-pigs (Minot), the Periwinkle (Bumpus), the 

 Crab, Carcinus (Weldon), and the human being (Bowditch, 

 Pearson, Roberts, and Boas). Vernon has shown that in the 

 Pluteus of Strongylocenlrotus the variability of the body-length 

 increases regularly up to the fifth day, and then decreases 

 regularly again to the sixteenth day. So FischeFs measurements 

 of Duck embryos seem to establish a greater variability in 

 younger than in older stages. This is true of the whole length, 

 the head (as far back as the first somite), the hand and trunk 

 together, and the total length exclusive of the primitive streak. 

 The data are, however, too few to be treated statistically; the 

 variability can only be roughly estimated from the extent of the 

 limits within which the part varies at each stage. 



Minot, who expresses the variability of guinea-pigs by the 

 difference between the mean weight and the mean weight of the 

 individuals above, and of those below, the mean, likewise finds 

 that the range in variation diminishes with age, and further 

 that, in the case of the males, there is a period from about the 

 fourth to the ninth months when the variability is very much 

 less than at any other time. No such sudden fall is observed in 

 the female, only a steady diminution. 



A more satisfactory calculation of the alteration of variability 

 may be made from the measurements taken by Bumpus of the 

 ' ventricosity ' (ratio of breadth to length) of the shell of the 

 Periwinkle, LUtorina littorea. The series of observations is very 

 large, and includes both British and American forms. In the 

 accompanying table (Table V) the coefficients of variability (the 

 standard deviation expressed as a percentage of the mean) are 



