14 THE PHENOMENA OF LIFE 



formance of their functions. Certain bacteria grow only in the absence of 

 oxygen. 



There is, commonly, a difference in general chemical composition be- 

 tween vegetables and animals, even in their lowest forms; for associated 

 with the protoplasm of the former is a considerable amount of cellulose, a 

 substance closely allied to starch and containing carbon, hydrogen, and 

 oxygen only. The presence of starch in vegetable cells is very character- 

 istic, though, as we have seen above, it is not distinctive, and a substance, 

 glycogen, similar in composition to starch, is very common in the organs and 

 tissues of animals. 



Inherent power of movement is a quality which we so commonly consider 

 an essential indication of animal nature that it is difficult at first to conceive 

 of its existence in any other. The capability of simple motion is now known, 

 however, to exist in so many vegetable forms that it can no longer be held 

 as an essential distinction between them and animals, and ceases to be a mark 

 by which one can be distinguished from the other. Thus the zoospores of 

 many of the Cryptogams exhibit ciliary or ameboid movements of a like 

 kind to those seen in amebae; and even among the higher orders of plants, 

 many, e.g.,Dionxa muscipula (Venus's fly-trap), and Mimosa sensitiva (Sensi- 

 tive plant) exhibit such motion, either at regular times or on the applica- 

 tion of external irritation, as might lead one, were this fact taken by itself, to 

 regard them as sentient beings. Inherent power of movement, then, al- 

 though especially characteristic of animal nature, is, when taken by itself, no 

 proof of it. 



Cell Differentiation and the Functions of Organized Cells. As we 

 proceed upward in the scale of life from the unicellular organisms, we find 

 another phenomenon exhibited in the life history of the higher forms, namely, 

 that of development. The one-celled ameba comes into being derived from 

 a previous ameba; it manifests the properties and performs the functions of 

 its life which have been already enumerated. In the higher organisms it is 

 different. Each, indeed, begins as a single cell, but the cells which result 

 from division and subdivision do not form so many independent organisms, 

 but adhere in one differentiated community which ultimately forms the 

 complex but co-ordinated whole, in man the human body. 



Thus, from the ovum, or germ cell which forms the starting-point of ani- 

 mal life, in a comparatively short time there is formed a complete membrane 

 of cells, polyhedral in shape from mutual pressure, called the Blastoderm; 

 and this speedily differentiates into two and then into three layers, chiefly 

 from the rapid proliferation of the cells of the first single layer. These layers, 

 figure 13, are called the Epiblast, the Mesoblast, and the HypoUast. In the 

 further development of the animal it is found that from each of these layers is 

 produced a very definite part of the completed body. For example, from 

 the cells of the epiblast are derived, among other structures, the skin and the 



