240 Experimental Zoology 
as determinate and indeterminate, although the distinction is of 
only secondary value. 
Growth and differentiation are often spoken of as the two 
processes by which the embryo is transformed into the adult. 
The distinction is difficult to apply, for differentiation is often 
accompanied by growth and growth by differentiation. 
The most general definition of organic growth is that of in- 
crease in volume. Sachs has pointed out that an increase in 
volume alone does not necessarily mean growth, because it may 
be due simply to swelling, as when a piece of dead wood im- 
bibes water and becomes larger. He defines growth as an in- 
crease in volume accompanied by a change of form; yet a 
change in form is not always apparent when we have reason to 
think that growth has really occurred. If, however, we include 
in our definition of growth the idea of an increase in the volume 
of the living material, we arrive at a more satisfactory definition. 
Some examples of normal growth may serve to bring the phe- 
nomena before us in a more concrete form. 
Davenport has measured the amount of water and of dry sub- 
stance in the tadpoles of frogs at different stages of their develop- 
ment. His results are summarized in the following table: — 
aes Days AFTER | AVERAGE WEIGHT piso WEIGHT OF PER CENT 
HatcHInG tn Mo’s SaRerA NCE WATER oF WATER 
May 2 I 1.83 80 1.03 56 
3 2 2.00 83 1.17 59 
6 5 3-43 80 2.63 a7 
8 7 5.05 54 4.51 89 
10 9 10.40 72 9.68 93 
15 14 23.52 1.16 22.36 096 
June 10 41 IOI.0O 9.90 QI.10 go 
July 23 84 1989.90 247.99 1742.00 88 
During the segmentation stages of the egg and during the 
early period of formation of the embryo (not included in the 
table) the increase in size of the embryo is not very great. After 
