1 68 ESSENTIALS OF BIOLOGY 



Organisms grow by selecting materials from their media and by 

 reassembling these into the materials of the tissues of their bodies. 

 It is difficult (and perhaps impossible) to find an analogy to this 

 process among inanimate things. 



Thus a plant grows by taking CO 2 from the air and water and 

 mineral salts from the soil. It reassembles, chemically, these 

 substances as carbohydrate, fats and proteins and it further 

 assembles these synthesized substances as wood-vessels, leaves, 

 buds, etc. The materials that are added, in growth, to the body 

 of an organism are therefore different from the materials that are 

 present in the medium. By " materials " we mean, of course, 

 the chemical compounds, for the chemical elements in the 

 organic body existed in, and were taken from the environing 

 media. 



6o«. Simple Organic Growth. For a short interval in the 

 life of an animal growth is apparently simple. Thus a boy of 

 about 18 years of age may grow for several years in such a way 

 that all the proportions of the body and limbs may remain almost 

 the same, nevertheless his height and weight may increase. In 

 such a case the body grows while retaining the same form. The 

 growth has occurred after the period of tissue and organic 

 differentiation. 



dob. Organic Growth with Differentiation. In general 

 the growth of an organism is accompanied by differentiation. 

 Beginning with the fertilized ovum the embryo increases in size, 

 but as it increases there occurs an increasing complexity of its 

 parts. The organ-rudiments form and then the tissues take on 

 their definite structures. The external form of the body may 

 change in a most striking way — even by a definite " metamor- 

 phosis," as in the case of the transformation of the tadpole into 

 the little frog. Even when the growth occurs in a direct way, 

 as in the case of man, the proportions of head, trunk and limbs 

 change rapidly during the juvenescent phase of individual life. 



Therefore organic growth is what we shall study, later on, as 

 development. In this process an internal agency remains the same 

 in all phases of life, but this agency — which is called the " specific 

 organization " — acts in such a way that the growing body passes 

 through a series of marked changes, or differentiations. If we 

 knew only the tadpole and the fully developed little frog but were 

 quite ignorant of the future of the tadpole and of the past of the 



