II. 2 



GROWTH 



67 



The decline of the growth-rate may also be seen in Pott's 

 weighings of the Chick embryo before hatching (Table I) and 

 Minot's weights of young chickens. It appears from these that 

 the daily percentage increment is 919% at the beginning of the 

 third day of incubation, 189% at the end of the fourth day; at 

 this point there is a sudden drop to 40 %, which is still the rate of 

 growth after eleven days of incubation ; eight days after hatch- 

 ing the rate is 9 % in the male, not quite 9 % in the female, and 

 then comes a period of more or less gradual decline, until when 

 the chicken is 342 days old it is able to add less than 0-5 % to its 

 weight per diem. 



Semper's observations on the pond-snail, Limnaea, and Vernon's 

 on the sea-urchin, St-rongylocentrotms, are other examples which 



.402 



.205 



24% 



20 



60 



FIG. 39. Daily percentage increments of weight in tadpoles : the con- 

 tinuous line (a) gives the whole weight, the broken line (6) the dry 

 weight. (After Davenport, 1899.) 



may be mentioned. The results of these authors are shown in 

 the accompanying charts (Fig. 38). Their measurements are of 

 lengths, not of weights. 



So far we have found no exception to the law of the decline 

 in the rate of growth as development proceeds. Davenport's 

 measurements of tadpoles will not, however, conform to the 

 generalization. As the figure shows (Fig. 39), the daily per- 

 centage increments, whether of the whole weight or of the 

 weight of dry substance only, first rise abruptly, then descend 

 and then rise again. An explanation of this anomaly may 

 possibly be found in the fact that Davenport's measurements are 



F 2, 



