I GROWTH OF THE GERM-CELLS 15 



also depends upon the absorption of water, as Davenport's 

 investigations upon tadpoles have shown. The percentage of 

 water in a tadpole's body, for example, increases during the 

 first fortnight after hatching from 50 % to over 80 % : after 

 that the water percentage decreases slightly. 



The same high proportion of water in the tissues in early 

 stages, followed later by a decline, has been demonstrated for 

 other embryos, for example the human embryo, as may be 

 gathered from the sixth column in the accompanying table 

 (Table I). At the present moment, however, the main interest 

 in growth undoubtedly centres in the way in which the rate 

 of growth alters during the progress of development. 



There is some difference of opinion as to the method by 

 which this rate should be measured. 



The table gives the actual weight (x) at certain times, the 

 increments from time to time (Ax), the average increments 



f > 



per unit of time for each of these intervals ( - ), and the 



percentage increments, or these increments expressed as a 

 percentage of the weight at the beginning of the interval 



r X -Y It is this last magnitude which Minot has 



proposed to employ as the measure of growth-rate, and if the 

 figures in the fifth column of the table be examined it will be 

 seen at once that the rate is high initially, diminishes abruptly 

 at first, thereafter more slowly to a final minimal value. 



The same abrupt descent from an initial high value, followed 

 by a more gradual decline, can be seen in many other cases ; for 

 example, in the figures for the change in the post-natal growth- 

 rate of man (Table II). There is a slight but temporary rise 

 about the time of puberty. The graph constructed from the 

 figures resembles therefore a logarithmic graph or curve. 



The parts of the body obey the same law as the whole, 

 although of course all the parts are not growing at the same 

 rate. The latter may be illustrated by the changes undergone 

 by the index-value of the parts (the dimension of the part 

 expressed as a percentage of that of the whole) during the 

 course of development. 



