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: — 



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 



