11.2 



GROWTH 



59 



It would thus appear that during the period of most rapid 

 growth, growth is effected by imbibition of water rather than 

 by assimilation, since the weight of dry substance in the tadpole 

 during this period does not increase at all. 



In later development the proportion of water slowly falls. 

 This may be seen not only in Davenport's table of the growth 

 of Frogs but in the data furnished by Potts for the Chick and by 



IOO'/A 



Diiys to 



30 



FIG. 34. Curve showing change in percentage of water in Frog tad- 

 poles from the first to the eighty-fourth day after hatching. Abscissae, 

 days ; ordinates, percentages. (After Davenport, from Korschelt and 

 Heider.) 



Fehling for the human embryo. These data are given in the 

 accompanying tables (Tables I, II). The percentage of water, 

 at first high, slowly falls in both cases ; conversely, the per- 

 centage of other substances increases. 



' These results indicate that during later development growth 

 is largely effected by excessive assimilation or by storing up 

 formed substance ' (Davenport). 



There are other external agencies by which growth may be 

 affected in various ways such as heat, light, and atmospheric 

 pressure. These will be discussed in another chapter. For the 

 present let us confine our attention to certain features which are 

 characteristic of growth in general, of the growth of the animal 

 organism under normal conditions. These are the changes that 



